From 28ae97fa45c15b677ee761efba5eafef3e06eb25 Mon Sep 17 00:00:00 2001
From: alex <alexander.heuwes@sames.com>
Date: Mon, 6 Jul 2026 20:16:51 +0200
Subject: [PATCH] safety commit

---
 include/config.h             |  22 ++-
 include/fan_step_response.h  | 268 +++++++++++++++++++++++++++++++++++
 include/thermal_controller.h | 125 ++++++++++++----
 include/tuning_store.h       |  10 +-
 scripts/dryer_tui.py         |  32 ++++-
 scripts/fan_test.py          | 147 +++++++++++++++++++
 scripts/step_response.py     | 169 ++++++++++++++++++++++
 src/main.cpp                 |  67 +++++++++
 8 files changed, 806 insertions(+), 34 deletions(-)
 create mode 100644 include/fan_step_response.h
 create mode 100644 scripts/fan_test.py
 create mode 100644 scripts/step_response.py

diff --git a/include/config.h b/include/config.h
index ae068a7..598050b 100644
--- a/include/config.h
+++ b/include/config.h
@@ -25,7 +25,10 @@ static const float TARGET_TEMP_C = 0.0f; // power-on default: idle (heater off)
 static const float AUTOTUNE_DEFAULT_TEMP_C = 40.0f; // autotune when no temp given and idle
 static const float TARGET_MIN_C = 0.0f;   // 0 = idle (heater off, fan at idle speed)
 static const float TARGET_MAX_C = 80.0f;
-static const float OVERTEMP_FRACTION = 0.05f; // hard cutoff at target * 1.05
+// Absolute max corner temp — heater off + full fan. Decoupled from PID target so you can
+// run target 50–55 while tuning with headroom for hot corners (ABS in chamber).
+static const float EMERGENCY_MAX_TEMP_C = 70.0f;
+static const float CORNER_STOP_MARGIN_C = 3.0f; // taper heater when max within this of emergency
 
 // PID on chamber average
 static const float PID_KP = 4.0f;
@@ -67,6 +70,9 @@ static const float FAN_OFF_BELOW_TARGET_C = 8.0f; // no heat-up fan when avg thi
 static const float FAN_RAMP_BELOW_TARGET_C = 15.0f; // fan ramps in between this and FAN_OFF_BELOW
 static const uint8_t FAN_MIX_MAX_PWM = 200;   // ~78 % — cap for spread-driven mixing
 static const uint8_t FAN_MAX_PWM = 255;       // failsafe / over-temp only
+// Most 24 V MOSFET modules are active-low (pin LOW = fan on). If off/speed seem wrong,
+// try flipping this and reflash. Test: `fan test 0` (off) vs `fan test 200` vs `fan test 255`.
+static const bool FAN_PWM_INVERT = true;
 
 // Corner mixing — moderate airflow; full speed reserved for safety
 static const float SPREAD_DEADBAND_C = 0.5f;
@@ -84,6 +90,20 @@ static const uint32_t AUTOTUNE_RELAY_STALL_MS = 1500000UL;      // 25 min in rel
 static const uint32_t AUTOTUNE_SESSION_TIMEOUT_MS = 3600000UL;   // 60 min total
 static const uint32_t AUTOTUNE_RELAY_PERIOD_MAX_MS = 2400000UL;
 
+// Fan step-response — open-loop heater, fan PWM steps (command: stepresp)
+static const float STEPRESP_DEFAULT_TEMP_C = 45.0f;
+static const float STEPRESP_DEFAULT_HEATER_PCT = 35.0f;
+static const float STEPRESP_MIN_HEATER_PCT = 10.0f;
+static const float STEPRESP_MAX_HEATER_PCT = 70.0f;
+static const float STEPRESP_PREHEAT_BAND_C = 2.0f;
+static const uint32_t STEPRESP_PREHEAT_TIMEOUT_MS = 1200000UL;  // 20 min
+static const uint32_t STEPRESP_BASELINE_MS = 120000UL;         // 2 min fan-off baseline
+static const uint32_t STEPRESP_STEP_HOLD_MS = 300000UL;        // 5 min per fan level
+static const uint32_t STEPRESP_LOG_INTERVAL_MS = 1000UL;
+static const uint8_t STEPRESP_FAN_STEPS[] = {0, 77, 140, 200, 255};
+static const uint8_t STEPRESP_FAN_STEP_COUNT =
+    sizeof(STEPRESP_FAN_STEPS) / sizeof(STEPRESP_FAN_STEPS[0]);
+
 // ---------------------------------------------------------------------------
 // Timing
 // ---------------------------------------------------------------------------
diff --git a/include/fan_step_response.h b/include/fan_step_response.h
new file mode 100644
index 0000000..c6a9f51
--- /dev/null
+++ b/include/fan_step_response.h
@@ -0,0 +1,268 @@
+#pragma once
+
+#include <Arduino.h>
+
+#include "config.h"
+
+class FanStepResponse {
+public:
+  enum class Phase : uint8_t { Idle, Preheat, Baseline, StepHold, Done, Failed };
+
+  FanStepResponse()
+      : phase_(Phase::Idle),
+        targetC_(STEPRESP_DEFAULT_TEMP_C),
+        heaterPct_(STEPRESP_DEFAULT_HEATER_PCT),
+        stepIndex_(0),
+        sessionStartMs_(0),
+        phaseStartMs_(0),
+        lastLogMs_(0),
+        lastAvgC_(0.0f) {}
+
+  Phase phase() const { return phase_; }
+
+  bool isActive() const {
+    return phase_ == Phase::Preheat || phase_ == Phase::Baseline || phase_ == Phase::StepHold;
+  }
+
+  uint32_t elapsedMs(uint32_t nowMs) const {
+    if (sessionStartMs_ == 0) {
+      return 0;
+    }
+    return nowMs - sessionStartMs_;
+  }
+
+  uint8_t stepIndex() const { return stepIndex_; }
+
+  uint8_t stepCount() const { return STEPRESP_FAN_STEP_COUNT; }
+
+  float targetC() const { return targetC_; }
+
+  float heaterPct() const { return heaterPct_; }
+
+  uint8_t currentFanPwm() const {
+    if (stepIndex_ >= STEPRESP_FAN_STEP_COUNT) {
+      return 0;
+    }
+    return STEPRESP_FAN_STEPS[stepIndex_];
+  }
+
+  const char *phaseName() const {
+    switch (phase_) {
+    case Phase::Preheat:
+      return "preheat";
+    case Phase::Baseline:
+      return "baseline";
+    case Phase::StepHold:
+      return "step";
+    default:
+      return "";
+    }
+  }
+
+  bool start(float targetC, float heaterPct) {
+    if (targetC < 25.0f || targetC > TARGET_MAX_C) {
+      return false;
+    }
+    if (heaterPct < STEPRESP_MIN_HEATER_PCT || heaterPct > STEPRESP_MAX_HEATER_PCT) {
+      return false;
+    }
+
+    targetC_ = targetC;
+    heaterPct_ = heaterPct;
+    stepIndex_ = 0;
+    sessionStartMs_ = millis();
+    phaseStartMs_ = sessionStartMs_;
+    lastLogMs_ = 0;
+    lastAvgC_ = 0.0f;
+    phase_ = Phase::Preheat;
+
+    Serial.print(F("stepresp: preheat to "));
+    Serial.print(targetC_ - STEPRESP_PREHEAT_BAND_C, 1);
+    Serial.print(F("-"));
+    Serial.print(targetC_, 1);
+    Serial.print(F("C avg, heater="));
+    Serial.print(heaterPct_, 0);
+    Serial.println(F("% fan=0"));
+    return true;
+  }
+
+  void abort() {
+    if (isActive()) {
+      Serial.println(F("stepresp: cancelled"));
+    }
+    phase_ = Phase::Idle;
+    sessionStartMs_ = 0;
+  }
+
+  void reset() {
+    phase_ = Phase::Idle;
+    sessionStartMs_ = 0;
+  }
+
+  bool update(float avgTempC, float maxTempC, float spreadC, uint32_t nowMs, float &heaterDutyOut,
+              uint8_t &fanPwmOut) {
+    heaterDutyOut = 0.0f;
+    fanPwmOut = 0;
+
+    if (phase_ == Phase::Idle || phase_ == Phase::Done || phase_ == Phase::Failed) {
+      return false;
+    }
+
+    if (maxTempC >= EMERGENCY_MAX_TEMP_C) {
+      fail(F("stepresp: abort — max sensor at safety limit"));
+      return false;
+    }
+
+    if (phase_ == Phase::Preheat) {
+      fanPwmOut = 0;
+      if (nowMs - phaseStartMs_ > STEPRESP_PREHEAT_TIMEOUT_MS) {
+        fail(F("stepresp: abort — preheat timeout"));
+        return false;
+      }
+      if (avgTempC >= targetC_ - STEPRESP_PREHEAT_BAND_C) {
+        enterBaseline(nowMs);
+      } else {
+        heaterDutyOut = heaterPct_;
+      }
+      return true;
+    }
+
+    heaterDutyOut = heaterPct_;
+    fanPwmOut = currentFanPwm();
+
+    if (phase_ == Phase::Baseline) {
+      if (nowMs - phaseStartMs_ >= STEPRESP_BASELINE_MS) {
+        advanceStep(nowMs);
+      }
+      return true;
+    }
+
+    if (phase_ == Phase::StepHold) {
+      if (nowMs - phaseStartMs_ >= STEPRESP_STEP_HOLD_MS) {
+        if (stepIndex_ + 1 >= STEPRESP_FAN_STEP_COUNT) {
+          finish(nowMs, avgTempC, spreadC);
+        } else {
+          ++stepIndex_;
+          enterStepHold(nowMs, true);
+        }
+      }
+      return true;
+    }
+
+    return false;
+  }
+
+  void logIfDue(const float *sensorTemps, const bool *sensorValid, uint8_t sensorCount,
+                float avgTempC, float minTempC, float maxTempC, float spreadC, uint32_t nowMs) {
+    if (!isActive()) {
+      return;
+    }
+    if (lastLogMs_ != 0 && nowMs - lastLogMs_ < STEPRESP_LOG_INTERVAL_MS) {
+      return;
+    }
+    lastLogMs_ = nowMs;
+    lastAvgC_ = avgTempC;
+
+    Serial.print(F("sr,"));
+    Serial.print(nowMs);
+    Serial.print(',');
+    Serial.print(phaseName());
+    Serial.print(',');
+    Serial.print(stepIndex_);
+    Serial.print('/');
+    Serial.print(STEPRESP_FAN_STEP_COUNT);
+    Serial.print(',');
+    Serial.print(heaterPct_, 0);
+    Serial.print(',');
+    Serial.print(currentFanPwm());
+    Serial.print(',');
+    Serial.print(avgTempC, 2);
+    Serial.print(',');
+    Serial.print(minTempC, 2);
+    Serial.print(',');
+    Serial.print(maxTempC, 2);
+    Serial.print(',');
+    Serial.print(spreadC, 2);
+
+    for (uint8_t i = 0; i < sensorCount; ++i) {
+      Serial.print(',');
+      if (sensorValid[i]) {
+        Serial.print(sensorTemps[i], 2);
+      }
+    }
+
+    Serial.println();
+  }
+
+private:
+  void enterBaseline(uint32_t nowMs) {
+    phase_ = Phase::Baseline;
+    phaseStartMs_ = nowMs;
+    stepIndex_ = 0;
+    Serial.print(F("stepresp: baseline fan="));
+    Serial.print(currentFanPwm());
+    Serial.print(F(" for "));
+    Serial.print(STEPRESP_BASELINE_MS / 1000UL);
+    Serial.println(F("s"));
+  }
+
+  void enterStepHold(uint32_t nowMs, bool isStep) {
+    phase_ = Phase::StepHold;
+    phaseStartMs_ = nowMs;
+    Serial.print(F("stepresp: "));
+    if (isStep) {
+      Serial.print(F("step "));
+    }
+    Serial.print(stepIndex_ + 1);
+    Serial.print(F("/"));
+    Serial.print(STEPRESP_FAN_STEP_COUNT);
+    Serial.print(F(" fan="));
+    Serial.print(currentFanPwm());
+    Serial.print(F(" ("));
+    Serial.print((currentFanPwm() * 100) / 255);
+    Serial.print(F("%) hold "));
+    Serial.print(STEPRESP_STEP_HOLD_MS / 1000UL);
+    Serial.println(F("s"));
+  }
+
+  void advanceStep(uint32_t nowMs) {
+    if (STEPRESP_FAN_STEP_COUNT <= 1) {
+      finish(nowMs, lastAvgC_, 0.0f);
+      return;
+    }
+    stepIndex_ = 1;
+    enterStepHold(nowMs, true);
+  }
+
+  void finish(uint32_t nowMs, float avgTempC, float spreadC) {
+    phase_ = Phase::Done;
+    Serial.print(F("stepresp: done in "));
+    Serial.print((nowMs - sessionStartMs_) / 1000UL);
+    Serial.println(F("s"));
+    Serial.print(F("  target="));
+    Serial.print(targetC_, 1);
+    Serial.print(F("C heater="));
+    Serial.print(heaterPct_, 0);
+    Serial.print(F("% final avg="));
+    Serial.print(avgTempC, 1);
+    Serial.print(F("C spread="));
+    Serial.print(spreadC, 1);
+    Serial.println(F("C"));
+    Serial.println(F("  parse sr,... lines for step response (fan PWM vs temp)"));
+  }
+
+  void fail(const __FlashStringHelper *reason) {
+    Serial.println(reason);
+    phase_ = Phase::Failed;
+    sessionStartMs_ = 0;
+  }
+
+  Phase phase_;
+  float targetC_;
+  float heaterPct_;
+  uint8_t stepIndex_;
+  uint32_t sessionStartMs_;
+  uint32_t phaseStartMs_;
+  uint32_t lastLogMs_;
+  float lastAvgC_;
+};
diff --git a/include/thermal_controller.h b/include/thermal_controller.h
index f007f05..7bbdcf2 100644
--- a/include/thermal_controller.h
+++ b/include/thermal_controller.h
@@ -3,6 +3,7 @@
 #include <Arduino.h>
 
 #include "config.h"
+#include "fan_step_response.h"
 #include "pid_autotuner.h"
 #include "pid_controller.h"
 #include "settings_store.h"
@@ -10,11 +11,12 @@
 
 class ThermalController {
 public:
-  enum class HeaterBlock : uint8_t { None, Cutoff, Corner, Allow, Autotune };
+  enum class HeaterBlock : uint8_t { None, Cutoff, Corner, Allow, Autotune, StepResp };
 
   ThermalController()
       : pid_(PID_KP, PID_KI, PID_KD, 0.0f, 100.0f),
         autotuner_(),
+        stepresp_(),
         targetTempC_(TARGET_TEMP_C),
         heaterDutyPercent_(0.0f),
         heaterAllowancePercent_(100.0f),
@@ -40,6 +42,7 @@ public:
     pinMode(FAN_PIN, OUTPUT);
     pinMode(HEATER_PIN, OUTPUT);
     digitalWrite(HEATER_PIN, LOW);
+    writeFan(0);
 
     pid_.setSetpoint(targetTempC_);
     pid_.reset();
@@ -105,7 +108,7 @@ public:
   bool isAdaptive() const { return adaptiveEnabled_; }
 
   bool startAutotune(float setpointC) {
-    if (autotuner_.isActive()) {
+    if (autotuner_.isActive() || stepresp_.isActive()) {
       return false;
     }
     adaptiveEnabled_ = false;
@@ -128,6 +131,46 @@ public:
 
   float autotunePreheatTargetC() const { return autotuner_.preheatTargetC(); }
 
+  bool startStepResponse(float targetC, float heaterPct) {
+    if (autotuner_.isActive() || stepresp_.isActive()) {
+      return false;
+    }
+    stopFanTest();
+    adaptiveEnabled_ = false;
+    cutoffActive_ = false;
+    pid_.reset();
+    setTarget(targetC, false);
+    if (!stepresp_.start(targetC, heaterPct)) {
+      return false;
+    }
+    writeFan(0);
+    return true;
+  }
+
+  void stopStepResponse() {
+    stepresp_.abort();
+    writeFan(0);
+  }
+
+  bool isStepResponseActive() const { return stepresp_.isActive(); }
+
+  uint32_t stepResponseElapsedMs(uint32_t nowMs) const { return stepresp_.elapsedMs(nowMs); }
+
+  const char *stepResponsePhaseName() const { return stepresp_.phaseName(); }
+
+  uint8_t stepResponseStepIndex() const { return stepresp_.stepIndex(); }
+
+  uint8_t stepResponseStepCount() const { return stepresp_.stepCount(); }
+
+  float stepResponseHeaterPct() const { return stepresp_.heaterPct(); }
+
+  void logStepResponseIfDue(const float *sensorTemps, const bool *sensorValid, uint8_t sensorCount,
+                            float avgTempC, float minTempC, float maxTempC, float spreadC,
+                            uint32_t nowMs) {
+    stepresp_.logIfDue(sensorTemps, sensorValid, sensorCount, avgTempC, minTempC, maxTempC,
+                       spreadC, nowMs);
+  }
+
   bool commitAutotuneIfDone() {
     if (autotuner_.phase() != PidAutotuner::Phase::Done) {
       return false;
@@ -206,7 +249,7 @@ public:
     if (isIdle()) {
       return INFINITY;
     }
-    return targetTempC_ * (1.0f + OVERTEMP_FRACTION);
+    return EMERGENCY_MAX_TEMP_C;
   }
 
   bool isCutoffActive() const { return cutoffActive_; }
@@ -249,6 +292,8 @@ public:
       return "allow";
     case HeaterBlock::Autotune:
       return "autotune";
+    case HeaterBlock::StepResp:
+      return "stepresp";
     default:
       return "none";
     }
@@ -264,6 +309,11 @@ public:
         SPREAD_EMA_ALPHA * cornerSpreadC +
         (1.0f - SPREAD_EMA_ALPHA) * cornerSpreadC_;
 
+    if (stepresp_.isActive()) {
+      updateStepResponse(avgTempC, maxTempC, nowMs);
+      return;
+    }
+
     if (autotuner_.isActive()) {
       updateAutotune(avgTempC, maxTempC, nowMs);
       return;
@@ -296,6 +346,7 @@ public:
     applyFan(millis());
     pid_.reset();
     autotuner_.abort();
+    stepresp_.abort();
   }
 
   void forceHeaterOff() {
@@ -307,16 +358,51 @@ public:
 
   void writeFan(uint8_t pwm) {
     fanPwm_ = pwm;
-    if (pwm == 0) {
-      // Re-assert output and stop Timer0 PWM on D5 — analogWrite(0) can leave the pin driving
-      pinMode(FAN_PIN, OUTPUT);
-      digitalWrite(FAN_PIN, LOW);
-    } else {
-      analogWrite(FAN_PIN, pwm);
+    pinMode(FAN_PIN, OUTPUT);
+
+    if (FAN_PWM_INVERT) {
+      if (pwm == 0) {
+        digitalWrite(FAN_PIN, HIGH);
+        return;
+      }
+      if (pwm >= 254) {
+        digitalWrite(FAN_PIN, LOW);
+        return;
+      }
+      analogWrite(FAN_PIN, static_cast<uint8_t>(255 - pwm));
+      return;
     }
+
+    if (pwm == 0) {
+      digitalWrite(FAN_PIN, LOW);
+      return;
+    }
+    if (pwm >= 254) {
+      digitalWrite(FAN_PIN, HIGH);
+      return;
+    }
+    analogWrite(FAN_PIN, pwm);
   }
 
 private:
+  void updateStepResponse(float avgTempC, float maxTempC, uint32_t nowMs) {
+    float duty = 0.0f;
+    uint8_t fan = 0;
+    stepresp_.update(avgTempC, maxTempC, cornerSpreadC_, nowMs, duty, fan);
+
+    heaterDutyPercent_ = duty;
+    heaterAllowancePercent_ = duty;
+    heaterBlock_ = duty > 0.0f ? HeaterBlock::StepResp : HeaterBlock::None;
+    applyHeaterBurst(nowMs);
+    writeFan(fan);
+    lastHeaterUpdateMs_ = nowMs;
+
+    if (stepresp_.phase() == FanStepResponse::Phase::Done ||
+        stepresp_.phase() == FanStepResponse::Phase::Failed) {
+      stepresp_.reset();
+    }
+  }
+
   void updateAutotune(float avgTempC, float maxTempC, uint32_t nowMs) {
     float duty = 0.0f;
     uint8_t fan = FAN_HEAT_MIN_PWM;
@@ -439,22 +525,10 @@ private:
   }
 
   float maxHeatStopTemp(float avgTempC) const {
-    if (avgTempC >= targetTempC_) {
-      return targetTempC_;
+    if (!shouldLimitMaxCorner(avgTempC)) {
+      return EMERGENCY_MAX_TEMP_C;
     }
-
-    float stopAt = targetTempC_;
-    if (isBalancedChamber()) {
-      stopAt = targetTempC_ + BALANCED_MAX_ABOVE_TARGET_C;
-    } else {
-      stopAt = targetTempC_ + cornerSpreadC_ * SPREAD_HEADROOM_FACTOR + 1.0f;
-    }
-
-    const float cutoff = cutoffThreshold();
-    if (stopAt > cutoff) {
-      stopAt = cutoff;
-    }
-    return stopAt;
+    return EMERGENCY_MAX_TEMP_C - CORNER_STOP_MARGIN_C;
   }
 
   float allowanceFromMaxCorner(float maxTempC, float avgTempC) const {
@@ -614,7 +688,7 @@ private:
     }
 
     if (isIdle()) {
-      if (!sensorWarmValid_ || lastMaxTempC_ >= IDLE_AUTO_FAN_OFF_TEMP_C) {
+      if (sensorWarmValid_ && lastMaxTempC_ >= IDLE_AUTO_FAN_OFF_TEMP_C) {
         writeFan(FAN_MAX_PWM);
       } else if (fanIdleOverride_) {
         writeFan(FAN_IDLE_PWM);
@@ -635,6 +709,7 @@ private:
 
   PidController pid_;
   PidAutotuner autotuner_;
+  FanStepResponse stepresp_;
   float targetTempC_;
   float heaterDutyPercent_;
   float heaterAllowancePercent_;
diff --git a/include/tuning_store.h b/include/tuning_store.h
index d93c6ab..3a65088 100644
--- a/include/tuning_store.h
+++ b/include/tuning_store.h
@@ -5,15 +5,15 @@
 
 #include "config.h"
 
-static const uint16_t TUNING_MAGIC = 0xDA7A;
+static const uint16_t TUNING_MAGIC = 0xDA7B;
 static const int TUNING_EEPROM_ADDR = 0;
 
 struct TuningData {
   uint16_t magic = 0;
-  float kp = PID_KP;
-  float ki = PID_KI;
-  float kd = PID_KD;
-  uint8_t fanMixMax = FAN_MIX_MAX_PWM;
+  float heatKp = HEAT_PI_KP;
+  float heatKi = HEAT_PI_KI;
+  float mixKp = MIX_PI_KP;
+  float mixKi = MIX_PI_KI;
 };
 
 inline uint8_t tuningChecksum(const TuningData &data) {
diff --git a/scripts/dryer_tui.py b/scripts/dryer_tui.py
index b24cd92..3a35376 100644
--- a/scripts/dryer_tui.py
+++ b/scripts/dryer_tui.py
@@ -43,7 +43,7 @@ STATUS_RE = re.compile(
     r"htop=(?P<htop>\S+)\s+"
     r"hblk=(?P<hblk>\S+)\s+"
     r"ssr=(?P<ssr>on|off)\s+"
-    r"fan=(?P<fan>\S+)\s+"
+    r"fan=(?P<fan>\d+/255\(\d+%\)(?:\([^)]+\))?(?:\s+TEST)?)\s+"
     r"cutoff=(?P<cutoff_active>\S+)\s+"
     r"failsafe=(?P<failsafe>\S+)\s+"
     r"mode=(?P<mode>.+?)\s+sensors=\[(?P<sensors>.*)\]"
@@ -55,6 +55,10 @@ AUTOTUNE_MODE_RE = re.compile(
     r"autotune/(?P<phase>[\w-]+) (?P<elapsed>\d+)s (?P<cycles>\d+/\d+)cyc pre>=(?P<pre>\d+)C"
 )
 
+STEPRESP_MODE_RE = re.compile(
+    r"stepresp/(?P<phase>[\w-]+) (?P<elapsed>\d+)s step (?P<step>\d+/\d+) heat=(?P<heat>\d+)%"
+)
+
 
 def format_mode_line(mode: str) -> str:
     match = AUTOTUNE_MODE_RE.match(mode)
@@ -64,6 +68,13 @@ def format_mode_line(mode: str) -> str:
             f"Autotune {d['phase']}: {d['elapsed']}s elapsed, "
             f"{d['cycles']} cycles, preheat avg >= {d['pre']} C"
         )
+    match = STEPRESP_MODE_RE.match(mode)
+    if match:
+        d = match.groupdict()
+        return (
+            f"Step response {d['phase']}: {d['elapsed']}s, "
+            f"step {d['step']}, heater {d['heat']}%"
+        )
     return f"Mode: {mode}"
 
 
@@ -141,7 +152,7 @@ def apply_status(state: DryerState, data: dict) -> None:
     fan_raw = data["fan"]
     state.fan = fan_raw
     state.fan_note = ""
-    if fan_raw.endswith("(off)") or fan_raw.endswith("(cooldown)") or " TEST" in fan_raw:
+    if fan_raw.endswith("(off)") or fan_raw.endswith("(cooldown)") or fan_raw.endswith("(cmd-off)") or " TEST" in fan_raw:
         state.fan_note = fan_raw[fan_raw.find("(") :] if "(" in fan_raw else ""
     state.cutoff_active = data["cutoff_active"]
     state.failsafe = data["failsafe"]
@@ -409,7 +420,7 @@ def _draw_dashboard(stdscr, state: DryerState) -> None:
         stdscr,
         help_y,
         1,
-        "0 idle | t target | p presets | f fan | l log | a autotune | : cmd | q quit",
+        "0 idle | t target | p presets | f fan | l log | a autotune | r stepresp | : cmd | q quit",
         curses.A_DIM,
     )
     stdscr.refresh()
@@ -491,6 +502,21 @@ def _curses_main(stdscr, ser, log_dir: Path, auto_log_on: bool) -> int:
                 if value is not None:
                     cmd = "autotune" if value == "" else f"autotune {value}"
                     worker.send(cmd)
+            elif key == ord("r"):
+                temp = _prompt(stdscr, "Stepresp temp °C (Enter = 45)")
+                if temp is None:
+                    continue
+                heater = _prompt(stdscr, "Heater % (Enter = 35)")
+                if heater is None:
+                    continue
+                if temp == "" and heater == "":
+                    worker.send("stepresp")
+                elif heater == "":
+                    worker.send(f"stepresp {temp}")
+                elif temp == "":
+                    worker.send(f"stepresp 45 {heater}")
+                else:
+                    worker.send(f"stepresp {temp} {heater}")
             elif key == ord(":"):
                 value = _prompt(stdscr, "Command")
                 if value is not None and value != "":
diff --git a/scripts/fan_test.py b/scripts/fan_test.py
new file mode 100644
index 0000000..d2ead19
--- /dev/null
+++ b/scripts/fan_test.py
@@ -0,0 +1,147 @@
+#!/usr/bin/env python3
+"""Cycle fan speeds on the dryer for wiring / PWM verification.
+
+Uses the firmware `fan test <pwm>` command (heater stays off). Sends `target 0`
+first so the thermal loop is idle.
+
+Example:
+  ./fan_test.py
+  ./fan_test.py --pct 30 50 100 --interval 3 --loop
+"""
+
+from __future__ import annotations
+
+import argparse
+import re
+import sys
+import time
+
+from capture_csv import decode_line, open_serial, resolve_port
+
+OK_RE = re.compile(r"^OK ")
+ERR_RE = re.compile(r"^ERR ")
+FAN_STATUS_RE = re.compile(r"fan=(\d+)/255\((\d+)%\)")
+
+
+def pct_to_pwm(pct: int) -> int:
+    if pct < 0 or pct > 100:
+        raise ValueError(f"fan percent must be 0-100, got {pct}")
+    return round(pct * 255 / 100)
+
+
+def send_command(ser, command: str, timeout: float = 2.0) -> list[str]:
+    ser.write((command.strip() + "\n").encode("utf-8"))
+    ser.flush()
+    lines: list[str] = []
+    deadline = time.monotonic() + timeout
+    while time.monotonic() < deadline:
+        raw = ser.readline()
+        if not raw:
+            continue
+        line = decode_line(raw)
+        if not line:
+            continue
+        lines.append(line)
+        if OK_RE.match(line) or ERR_RE.match(line):
+            break
+    return lines
+
+
+def drain_status(ser, duration: float) -> str | None:
+    """Read serial for `duration` seconds; return last status fan field if seen."""
+    fan_field: str | None = None
+    deadline = time.monotonic() + duration
+    while time.monotonic() < deadline:
+        raw = ser.readline()
+        if not raw:
+            continue
+        line = decode_line(raw)
+        if not line or line.startswith("csv"):
+            continue
+        match = FAN_STATUS_RE.search(line)
+        if match:
+            fan_field = f"{match.group(1)}/255 ({match.group(2)}%)"
+        elif line.startswith("target="):
+            print(f"  status: {line}", flush=True)
+    return fan_field
+
+
+def main() -> int:
+    parser = argparse.ArgumentParser(description="Cycle fan PWM to verify fan control")
+    parser.add_argument(
+        "-p",
+        "--port",
+        help="Serial port (default: auto-detect)",
+    )
+    parser.add_argument("-b", "--baud", type=int, default=115200)
+    parser.add_argument(
+        "--pct",
+        type=int,
+        nargs="+",
+        default=[0, 30, 100, 200, 255],
+        metavar="PCT",
+        help="Fan speeds in percent (default: 0 30 100 200 255)",
+    )
+    parser.add_argument(
+        "--interval",
+        type=float,
+        default=5.0,
+        metavar="SEC",
+        help="Seconds to hold each step (default: 5)",
+    )
+    parser.add_argument(
+        "--loop",
+        action="store_true",
+        help="Repeat the sequence until Ctrl+C",
+    )
+    args = parser.parse_args()
+
+    port = resolve_port(args.port)
+    sequence = [(pct, pct_to_pwm(pct)) for pct in args.pct]
+
+    print(f"Port: {port}", file=sys.stderr)
+    print(
+        f"Sequence: {' -> '.join(str(p) + '%' for p, _ in sequence)} "
+        f"every {args.interval:g}s (heater off)",
+        file=sys.stderr,
+    )
+    print("Ctrl+C to stop\n", file=sys.stderr)
+
+    interrupted = False
+    with open_serial(port, args.baud) as ser:
+        ser.reset_input_buffer()
+
+        lines = send_command(ser, "target 0")
+        for line in lines:
+            print(line, flush=True)
+        if any(ERR_RE.match(line) for line in lines):
+            return 1
+
+        try:
+            while True:
+                for pct, pwm in sequence:
+                    print(f">>> fan test {pwm}  ({pct}%)", flush=True)
+                    lines = send_command(ser, f"fan test {pwm}")
+                    for line in lines:
+                        print(f"    {line}", flush=True)
+                    if any(ERR_RE.match(line) for line in lines):
+                        return 1
+
+                    reported = drain_status(ser, args.interval)
+                    if reported:
+                        print(f"    reported fan={reported}", flush=True)
+
+                if not args.loop:
+                    break
+        except KeyboardInterrupt:
+            interrupted = True
+            print("\nInterrupted", file=sys.stderr)
+        finally:
+            print(">>> fan test 0 (stop)", flush=True)
+            send_command(ser, "fan test 0")
+
+    return 130 if interrupted else 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())
diff --git a/scripts/step_response.py b/scripts/step_response.py
new file mode 100644
index 0000000..bff5bc1
--- /dev/null
+++ b/scripts/step_response.py
@@ -0,0 +1,169 @@
+#!/usr/bin/env python3
+"""Run fan step-response test and capture sr,... serial log lines to CSV.
+
+The firmware holds heater duty fixed, steps fan PWM, and logs temperature
+every second. Use the output to see how chamber temp responds to fan changes.
+
+Example:
+  ./step_response.py
+  ./step_response.py --temp 45 --heater 35 -o logs/stepresp.csv
+"""
+
+from __future__ import annotations
+
+import argparse
+import re
+import sys
+import time
+from datetime import datetime, timezone
+from pathlib import Path
+
+from capture_csv import decode_line, open_serial, resolve_port
+
+SR_RE = re.compile(
+    r"^sr,(?P<ms>\d+),(?P<phase>\w+),(?P<step>\d+/\d+),"
+    r"(?P<heater>\d+),(?P<fan>\d+),"
+    r"(?P<avg>[\d.]+),(?P<min>[\d.]+),(?P<max>[\d.]+),(?P<spread>[\d.]+)"
+    r"(?:,(?P<temps>.*))?$"
+)
+DONE_RE = re.compile(r"^stepresp: done")
+FAIL_RE = re.compile(r"^stepresp: abort")
+
+HEADER = (
+    "wall_time,ms,phase,step,heater_pct,fan_pwm,fan_pct,avg_c,min_c,max_c,spread_c,"
+    "ch2_t,ch3_t,ch4_t,ch5_t"
+)
+
+
+def fan_pct(pwm: int) -> int:
+    return (pwm * 100) // 255
+
+
+def send_command(ser, command: str, timeout: float = 3.0) -> list[str]:
+    ser.write((command.strip() + "\n").encode("utf-8"))
+    ser.flush()
+    lines: list[str] = []
+    deadline = time.monotonic() + timeout
+    while time.monotonic() < deadline:
+        raw = ser.readline()
+        if not raw:
+            continue
+        line = decode_line(raw)
+        if not line:
+            continue
+        lines.append(line)
+        if line.startswith("OK ") or line.startswith("ERR "):
+            break
+    return lines
+
+
+def parse_sr_line(line: str) -> dict | None:
+    match = SR_RE.match(line)
+    if not match:
+        return None
+    data = match.groupdict()
+    temps = data.pop("temps") or ""
+    channels = (temps.split(",") + ["", "", "", ""])[:4]
+    data["ch2_t"], data["ch3_t"], data["ch4_t"], data["ch5_t"] = channels
+    data["fan_pwm"] = data.pop("fan")
+    data["heater_pct"] = data.pop("heater")
+    return data
+
+
+def main() -> int:
+    parser = argparse.ArgumentParser(description="Capture fan step-response data")
+    parser.add_argument("-p", "--port", help="Serial port (default: auto-detect)")
+    parser.add_argument("-b", "--baud", type=int, default=115200)
+    parser.add_argument("--temp", type=float, default=45.0, help="Target temperature (C)")
+    parser.add_argument("--heater", type=float, default=35.0, help="Fixed heater duty (%%)")
+    parser.add_argument(
+        "-o",
+        "--output",
+        type=Path,
+        help="Output CSV (default: logs/stepresp_YYYYMMDD_HHMMSS.csv)",
+    )
+    args = parser.parse_args()
+
+    port = resolve_port(args.port)
+    out = args.output
+    if out is None:
+        out = Path("logs") / f"stepresp_{datetime.now():%Y%m%d_%H%M%S}.csv"
+
+    print(f"Port: {port}", file=sys.stderr)
+    print(f"Output: {out}", file=sys.stderr)
+    print(f"Command: stepresp {args.temp:g} {args.heater:g}", file=sys.stderr)
+    print("Ctrl+C to stop\n", file=sys.stderr)
+
+    out.parent.mkdir(parents=True, exist_ok=True)
+    row_count = 0
+
+    with open_serial(port, args.baud) as ser, out.open("w", encoding="utf-8") as fh:
+        fh.write(HEADER + "\n")
+        ser.reset_input_buffer()
+
+        lines = send_command(ser, f"stepresp {args.temp:g} {args.heater:g}")
+        for line in lines:
+            print(line, flush=True)
+        if any(line.startswith("ERR ") for line in lines):
+            return 1
+
+        try:
+            while True:
+                raw = ser.readline()
+                if not raw:
+                    continue
+                line = decode_line(raw)
+                if not line:
+                    continue
+
+                if line.startswith("sr,"):
+                    data = parse_sr_line(line)
+                    if data is None:
+                        print(f"WARN: bad sr line: {line}", file=sys.stderr)
+                        continue
+                    pwm = int(data["fan_pwm"])
+                    wall = datetime.now(timezone.utc).isoformat(timespec="seconds")
+                    row = [
+                        wall,
+                        data["ms"],
+                        data["phase"],
+                        data["step"],
+                        data["heater_pct"],
+                        str(pwm),
+                        str(fan_pct(pwm)),
+                        data["avg"],
+                        data["min"],
+                        data["max"],
+                        data["spread"],
+                        data["ch2_t"],
+                        data["ch3_t"],
+                        data["ch4_t"],
+                        data["ch5_t"],
+                    ]
+                    fh.write(",".join(row) + "\n")
+                    fh.flush()
+                    row_count += 1
+                    if row_count % 30 == 0:
+                        print(
+                            f"  {data['phase']} step {data['step']} "
+                            f"fan={pwm} avg={data['avg']}C spread={data['spread']}C",
+                            flush=True,
+                        )
+                    continue
+
+                if DONE_RE.match(line) or FAIL_RE.match(line):
+                    print(line, flush=True)
+                    break
+
+                if line.startswith("stepresp:"):
+                    print(line, flush=True)
+        except KeyboardInterrupt:
+            print("\nStopping…", file=sys.stderr)
+            send_command(ser, "stepresp stop")
+
+    print(f"Wrote {row_count} rows to {out}", file=sys.stderr)
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())
diff --git a/src/main.cpp b/src/main.cpp
index 0a889a0..80a19fd 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -146,6 +146,8 @@ void printStatus(float avgTemp, float minTemp, float maxTemp) {
     Serial.print(F("(off)"));
   } else if (thermal.isIdleCooling()) {
     Serial.print(F("(cooldown)"));
+  } else if (thermal.fanPwm() == 0) {
+    Serial.print(F("(cmd-off)"));
   }
   Serial.print(F(" cutoff="));
   Serial.print(thermal.isCutoffActive() ? F("YES") : F("no"));
@@ -164,6 +166,18 @@ void printStatus(float avgTemp, float minTemp, float maxTemp) {
     Serial.print(F("cyc pre>="));
     Serial.print(thermal.autotunePreheatTargetC(), 0);
     Serial.print(F("C"));
+  } else if (thermal.isStepResponseActive()) {
+    Serial.print(F("stepresp/"));
+    Serial.print(thermal.stepResponsePhaseName());
+    Serial.print(F(" "));
+    Serial.print(thermal.stepResponseElapsedMs(millis()) / 1000UL);
+    Serial.print(F("s step "));
+    Serial.print(thermal.stepResponseStepIndex() + 1);
+    Serial.print(F("/"));
+    Serial.print(thermal.stepResponseStepCount());
+    Serial.print(F(" heat="));
+    Serial.print(thermal.stepResponseHeaterPct(), 0);
+    Serial.print(F("%"));
   } else if (thermal.isAdaptive()) {
     Serial.print(F("learned"));
   } else {
@@ -199,6 +213,8 @@ void printHelp() {
   Serial.println(F("  fan test N  set fan PWM 0-255 for 15s (verify wiring)"));
   Serial.println(F("  autotune [C] learn PID (default: 40C when idle)"));
   Serial.println(F("  autotune stop"));
+  Serial.println(F("  stepresp [C] [heater%] fan step response (default: 45C 35%)"));
+  Serial.println(F("  stepresp stop"));
   Serial.println(F("  pid         show PID / adaptive status"));
   Serial.println(F("  pid default reset to factory PID"));
   Serial.println(F("  status      print current readings"));
@@ -328,6 +344,45 @@ void processSerialLine(const char *line) {
     return;
   }
 
+  if (strncmp(line, "stepresp", 8) == 0) {
+    if (strcmp(line, "stepresp stop") == 0) {
+      thermal.stopStepResponse();
+      Serial.println(F("OK stepresp cancelled"));
+      return;
+    }
+
+    float tempC = STEPRESP_DEFAULT_TEMP_C;
+    float heaterPct = STEPRESP_DEFAULT_HEATER_PCT;
+    if (line[8] == ' ') {
+      const char *args = line + 9;
+      tempC = atof(args);
+      const char *space = strchr(args, ' ');
+      if (space != nullptr) {
+        heaterPct = atof(space + 1);
+      }
+    }
+
+    if (tempC < 25.0f || tempC > TARGET_MAX_C) {
+      Serial.println(F("ERR stepresp temperature must be 25-80 C"));
+      return;
+    }
+    if (heaterPct < STEPRESP_MIN_HEATER_PCT || heaterPct > STEPRESP_MAX_HEATER_PCT) {
+      Serial.print(F("ERR stepresp heater must be "));
+      Serial.print(STEPRESP_MIN_HEATER_PCT, 0);
+      Serial.print(F("-"));
+      Serial.print(STEPRESP_MAX_HEATER_PCT, 0);
+      Serial.println(F("%"));
+      return;
+    }
+
+    if (!thermal.startStepResponse(tempC, heaterPct)) {
+      Serial.println(F("ERR stepresp already running or autotune active"));
+      return;
+    }
+    Serial.println(F("OK stepresp started — open-loop heater, fan steps, ~25-35 min"));
+    return;
+  }
+
   if (strcmp(line, "pid") == 0 || strcmp(line, "pid show") == 0) {
     thermal.printTuning();
     return;
@@ -428,6 +483,18 @@ void loop() {
 
     if (!isnan(avgTemp) && !isnan(maxTemp) && !isnan(spread)) {
       thermal.update(avgTemp, maxTemp, spread, now);
+
+      float sensorTemps[SENSOR_COUNT];
+      bool sensorValid[SENSOR_COUNT];
+      for (uint8_t i = 0; i < SENSOR_COUNT; ++i) {
+        sensorTemps[i] = sensors[i].temperatureC;
+        sensorValid[i] = sensors[i].valid;
+      }
+      const float minTemp = minValidTemperature();
+      if (thermal.isStepResponseActive() && !isnan(minTemp)) {
+        thermal.logStepResponseIfDue(sensorTemps, sensorValid, SENSOR_COUNT, avgTemp, minTemp,
+                                     maxTemp, spread, now);
+      }
     } else {
       thermal.enterFailSafe();
       Serial.println(F("WARN: no valid sensor readings — heater off"));