diff --git a/include/config.h b/include/config.h
index 3658130..ae068a7 100644
--- a/include/config.h
+++ b/include/config.h
@@ -75,13 +75,14 @@ static const float SPREAD_EMA_ALPHA = 0.45f;
 
 // PID auto-tune (relay method) — run with: autotune 45
 static const float AUTOTUNE_HYSTERESIS_C = 0.4f;
-static const float AUTOTUNE_PREHEAT_BAND_C = 5.0f;
+static const float AUTOTUNE_PREHEAT_BAND_C = 3.0f;
 static const float AUTOTUNE_PREHEAT_DUTY = 100.0f;
-static const uint8_t AUTOTUNE_PREHEAT_FAN_PWM = 70; // low fan for entire autotune
-static const uint8_t AUTOTUNE_CYCLES_REQUIRED = 6;
+static const uint8_t AUTOTUNE_PREHEAT_FAN_PWM = 0; // fan off — maximize heat-up
+static const uint8_t AUTOTUNE_CYCLES_REQUIRED = 5;
 static const uint32_t AUTOTUNE_PREHEAT_TIMEOUT_MS = 1200000UL;  // 20 min
-static const uint32_t AUTOTUNE_SESSION_TIMEOUT_MS = 3600000UL;    // 60 min total
-static const uint32_t AUTOTUNE_RELAY_PERIOD_MAX_MS = 2400000UL;   // count periods up to 40 min
+static const uint32_t AUTOTUNE_RELAY_STALL_MS = 1500000UL;      // 25 min in relay, 0 cycles
+static const uint32_t AUTOTUNE_SESSION_TIMEOUT_MS = 3600000UL;   // 60 min total
+static const uint32_t AUTOTUNE_RELAY_PERIOD_MAX_MS = 2400000UL;
 
 // ---------------------------------------------------------------------------
 // Timing
diff --git a/include/pid_autotuner.h b/include/pid_autotuner.h
index c621efe..46c1262 100644
--- a/include/pid_autotuner.h
+++ b/include/pid_autotuner.h
@@ -24,6 +24,7 @@ public:
         spreadSamples_(0),
         cycleCount_(0),
         aboveSetpoint_(false),
+        useMaxSensorPv_(false),
         sessionStartMs_(0),
         phaseStartMs_(0),
         resultKp_(PID_KP),
@@ -47,12 +48,14 @@ public:
 
   float preheatTargetC() const { return setpointC_ - AUTOTUNE_PREHEAT_BAND_C; }
 
+  bool usesMaxSensor() const { return useMaxSensorPv_; }
+
   const char *phaseName() const {
     switch (phase_) {
     case Phase::Preheat:
       return "preheat";
     case Phase::Relay:
-      return "relay";
+      return useMaxSensorPv_ ? "relay-max" : "relay-avg";
     default:
       return "";
     }
@@ -124,8 +127,8 @@ public:
         fail(F("autotune: abort — preheat timeout"));
         return phase_;
       }
-      if (avgTempC >= preheatTargetC()) {
-        enterRelay(avgTempC, nowMs);
+      if (avgTempC >= preheatTargetC() || maxTempC >= setpointC_ - 2.0f) {
+        enterRelay(avgTempC, maxTempC, spreadC, nowMs);
       } else {
         heaterDutyOut = AUTOTUNE_PREHEAT_DUTY;
       }
@@ -137,27 +140,44 @@ public:
       return phase_;
     }
 
+    if (nowMs - sessionStartMs_ > AUTOTUNE_SESSION_TIMEOUT_MS) {
+      fail(F("autotune: abort — session timeout"));
+      return phase_;
+    }
+
+    if (cycleCount_ == 0 && nowMs - phaseStartMs_ > AUTOTUNE_RELAY_STALL_MS) {
+      Serial.print(F("autotune: relay stalled — avg "));
+      Serial.print(avgTempC, 1);
+      Serial.print(F("C max "));
+      Serial.print(maxTempC, 1);
+      Serial.println(F("C (spread too large for avg to cross setpoint?)"));
+      fail(F("autotune: abort — no oscillation"));
+      return phase_;
+    }
+
+    const float pv = useMaxSensorPv_ ? maxTempC : avgTempC;
+
     spreadSum_ += spreadC;
     ++spreadSamples_;
 
-    if (avgTempC > peakSinceCross_) {
-      peakSinceCross_ = avgTempC;
+    if (pv > peakSinceCross_) {
+      peakSinceCross_ = pv;
     }
-    if (avgTempC < valleySinceCross_) {
-      valleySinceCross_ = avgTempC;
+    if (pv < valleySinceCross_) {
+      valleySinceCross_ = pv;
     }
 
     bool heatOn = false;
-    if (avgTempC <= relayLow_) {
+    if (pv <= relayLow_) {
       heatOn = true;
-    } else if (avgTempC >= relayHigh_) {
+    } else if (pv >= relayHigh_) {
       heatOn = false;
     } else {
       heatOn = !aboveSetpoint_;
     }
     heaterDutyOut = heatOn ? 100.0f : 0.0f;
 
-    const bool nowAbove = avgTempC >= setpointC_;
+    const bool nowAbove = pv >= setpointC_;
     if (nowAbove != aboveSetpoint_) {
       onSetpointCrossing(nowMs);
       aboveSetpoint_ = nowAbove;
@@ -167,14 +187,18 @@ public:
   }
 
 private:
-  void enterRelay(float avgTempC, uint32_t nowMs) {
+  void enterRelay(float avgTempC, float maxTempC, float spreadC, uint32_t nowMs) {
     phase_ = Phase::Relay;
     phaseStartMs_ = nowMs;
-    aboveSetpoint_ = avgTempC >= setpointC_;
-    peakSinceCross_ = avgTempC;
-    valleySinceCross_ = avgTempC;
+    useMaxSensorPv_ = spreadC > GOOD_SPREAD_C;
+    const float pv = useMaxSensorPv_ ? maxTempC : avgTempC;
+    aboveSetpoint_ = pv >= setpointC_;
+    peakSinceCross_ = pv;
+    valleySinceCross_ = pv;
     lastCrossMs_ = 0;
-    Serial.print(F("autotune: relay test started ("));
+    Serial.print(F("autotune: relay "));
+    Serial.print(useMaxSensorPv_ ? F("max-sensor") : F("avg"));
+    Serial.print(F(" ("));
     Serial.print((nowMs - sessionStartMs_) / 1000UL);
     Serial.println(F("s preheat)"));
   }
@@ -191,6 +215,7 @@ private:
     spreadSamples_ = 0;
     cycleCount_ = 0;
     aboveSetpoint_ = false;
+    useMaxSensorPv_ = false;
   }
 
   void onSetpointCrossing(uint32_t nowMs) {
@@ -295,6 +320,7 @@ private:
   uint16_t spreadSamples_;
   uint8_t cycleCount_;
   bool aboveSetpoint_;
+  bool useMaxSensorPv_;
   uint32_t sessionStartMs_;
   uint32_t phaseStartMs_;
   float resultKp_;
diff --git a/scripts/capture_csv.py b/scripts/capture_csv.py
index c6ef151..45a4bb9 100755
--- a/scripts/capture_csv.py
+++ b/scripts/capture_csv.py
@@ -9,6 +9,7 @@
 from __future__ import annotations
 
 import argparse
+import re
 import sys
 import time
 from datetime import datetime, timezone
@@ -19,6 +20,118 @@ FALLBACK_HEADER = (
     "fan_pct,cutoff,failsafe,ch2_t,ch2_h,ch3_t,ch3_h,ch4_t,ch4_h,ch5_t,ch5_h"
 )
 
+SENSOR_CHANNELS = [2, 3, 4, 5]
+
+_FAN_PCT_RE = re.compile(r"\((\d+)%\)")
+_FAN_PWM_RE = re.compile(r"^(\d+)/")
+
+
+def fan_pct_from_status(fan: str) -> int:
+    match = _FAN_PCT_RE.search(fan)
+    if match:
+        return int(match.group(1))
+    match = _FAN_PWM_RE.match(fan)
+    if match:
+        return (int(match.group(1)) * 100) // 255
+    return 0
+
+
+def target_c_from_status(target: str) -> str:
+    if target.startswith("idle"):
+        return "0.0"
+    if target.endswith("C"):
+        return target[:-1]
+    return target
+
+
+def build_csv_payload_from_status(data: dict, ms: int | None = None) -> str:
+    if ms is None:
+        ms = int(time.time() * 1000)
+
+    sensors = {ch: (temp, hum) for ch, temp, hum in data.get("sensor_list", [])}
+    parts = [
+        str(ms),
+        target_c_from_status(data["target"]),
+        data["avg"],
+        data["min"],
+        data["max"],
+        data["spread"],
+        data["heatlim"],
+        data["heater"],
+        str(fan_pct_from_status(data.get("fan", "0"))),
+        "1" if data.get("cutoff_active") == "YES" else "0",
+        "1" if data.get("failsafe") == "YES" else "0",
+    ]
+    for ch in SENSOR_CHANNELS:
+        if str(ch) in sensors:
+            temp, hum = sensors[str(ch)]
+            if temp == "ERR":
+                parts.extend(["", ""])
+            else:
+                parts.extend([temp, hum])
+        else:
+            parts.extend(["", ""])
+    return ",".join(parts)
+
+
+class CsvSession:
+    """Deferred CSV writer — no empty file until the first row lands."""
+
+    def __init__(self, path: Path):
+        self.path = path
+        self._fh = None
+        self._header_written = False
+        self.row_count = 0
+
+    def _ensure_open(self) -> None:
+        if self._fh is None:
+            self.path.parent.mkdir(parents=True, exist_ok=True)
+            self._fh = self.path.open("w", encoding="utf-8")
+
+    def _write_header(self) -> None:
+        if not self._header_written:
+            self._ensure_open()
+            assert self._fh is not None
+            self._fh.write(FALLBACK_HEADER + "\n")
+            self._header_written = True
+
+    def write_device_line(self, line: str) -> None:
+        if line.startswith("csv_hdr,"):
+            self._ensure_open()
+            assert self._fh is not None
+            device_header = line[len("csv_hdr,") :]
+            self._fh.write("wall_time," + device_header + "\n")
+            self._header_written = True
+            self._fh.flush()
+            return
+        if not line.startswith("csv,"):
+            return
+        self._write_header()
+        assert self._fh is not None
+        wall_time = datetime.now(timezone.utc).isoformat(timespec="seconds")
+        self._fh.write(wall_time + "," + line[len("csv,") :] + "\n")
+        self._fh.flush()
+        self.row_count += 1
+
+    def write_status(self, data: dict) -> None:
+        self._write_header()
+        assert self._fh is not None
+        wall_time = datetime.now(timezone.utc).isoformat(timespec="seconds")
+        payload = build_csv_payload_from_status(data)
+        self._fh.write(wall_time + "," + payload + "\n")
+        self._fh.flush()
+        self.row_count += 1
+
+    def close(self) -> None:
+        if self._fh is not None:
+            self._fh.close()
+            self._fh = None
+        if self.row_count == 0 and self.path.exists():
+            try:
+                self.path.unlink()
+            except OSError:
+                pass
+
 
 def detect_serial_port() -> str | None:
     by_id = Path("/dev/serial/by-id")
@@ -93,36 +206,20 @@ def enable_dryer_logging(ser, retries: int = 3) -> None:
     print("WARN: did not see 'OK csv logging on' — continuing anyway", file=sys.stderr)
 
 
-def write_csv_row(fh, line: str, header_written: list[bool]) -> None:
-    if line.startswith("csv_hdr,"):
-        device_header = line[len("csv_hdr,") :]
-        fh.write("wall_time," + device_header + "\n")
-        header_written[0] = True
-        fh.flush()
-        return
-    if not line.startswith("csv,"):
-        return
-    if not header_written[0]:
-        fh.write(FALLBACK_HEADER + "\n")
-        header_written[0] = True
-    wall_time = datetime.now(timezone.utc).isoformat(timespec="seconds")
-    fh.write(wall_time + "," + line[len("csv,") :] + "\n")
-    fh.flush()
-
-
 def cmd_log(args: argparse.Namespace) -> int:
+    from dryer_tui import parse_status
+
     port = resolve_port(args.port)
     out = args.output
     if out is None:
         out = args.log_dir / f"dryer_{datetime.now():%Y%m%d_%H%M%S}.csv"
-    out.parent.mkdir(parents=True, exist_ok=True)
 
     print(f"Logging {port} -> {out}", file=sys.stderr)
     if args.auto_log_on:
         print("Will send 'log on' after connect", file=sys.stderr)
 
-    header_written = False
-    with open_serial(port, args.baud) as ser, out.open("w", encoding="utf-8") as fh:
+    session = CsvSession(out)
+    with open_serial(port, args.baud) as ser:
         if args.auto_log_on:
             enable_dryer_logging(ser)
 
@@ -130,32 +227,27 @@ def cmd_log(args: argparse.Namespace) -> int:
             try:
                 raw = ser.readline()
             except KeyboardInterrupt:
-                print("\nStopped.", file=sys.stderr)
+                print(f"\nStopped ({session.row_count} rows).", file=sys.stderr)
+                session.close()
                 return 0
 
             if not raw:
                 continue
 
             line = decode_line(raw)
-            if not line.startswith("csv_hdr,") and not line.startswith("csv,"):
-                if line:
-                    print(line)
+            parsed = parse_status(line)
+            if parsed:
+                session.write_status(parsed)
+                print(line)
                 continue
 
-            if line.startswith("csv_hdr,"):
-                device_header = line[len("csv_hdr,") :]
-                fh.write("wall_time," + device_header + "\n")
-                header_written = True
-                fh.flush()
+            if line.startswith("csv_hdr,") or line.startswith("csv,"):
+                session.write_device_line(line)
                 continue
 
-            if not header_written:
-                fh.write(FALLBACK_HEADER + "\n")
-                header_written = True
-
-            wall_time = datetime.now(timezone.utc).isoformat(timespec="seconds")
-            fh.write(wall_time + "," + line[len("csv,") :] + "\n")
-            fh.flush()
+            if line:
+                print(line)
+    return 0
 
 
 def cmd_tui(args: argparse.Namespace) -> int:
diff --git a/scripts/dryer_tui.py b/scripts/dryer_tui.py
index 18eab24..b24cd92 100644
--- a/scripts/dryer_tui.py
+++ b/scripts/dryer_tui.py
@@ -13,8 +13,8 @@ from datetime import datetime
 from pathlib import Path
 
 from capture_csv import (
+    CsvSession,
     decode_line,
-    write_csv_row,
 )
 
 PRESETS: list[tuple[str, float]] = [
@@ -52,7 +52,7 @@ STATUS_RE = re.compile(
 SENSOR_RE = re.compile(r"ch(\d+):([\d.]+)C/(\d+)%|ch(\d+):ERR")
 
 AUTOTUNE_MODE_RE = re.compile(
-    r"autotune/(?P<phase>\w+) (?P<elapsed>\d+)s (?P<cycles>\d+/\d+)cyc pre>=(?P<pre>\d+)C"
+    r"autotune/(?P<phase>[\w-]+) (?P<elapsed>\d+)s (?P<cycles>\d+/\d+)cyc pre>=(?P<pre>\d+)C"
 )
 
 
@@ -155,8 +155,7 @@ class SerialWorker:
         self.state = state
         self.lock = lock
         self.stop = threading.Event()
-        self._log_fh = None
-        self._header_written = [False]
+        self._csv: CsvSession | None = None
         self._thread: threading.Thread | None = None
 
     def start(self) -> None:
@@ -171,9 +170,9 @@ class SerialWorker:
         self.stop.set()
         if self._thread is not None:
             self._thread.join(timeout=1.5)
-        if self._log_fh is not None:
-            self._log_fh.close()
-            self._log_fh = None
+        if self._csv is not None:
+            self._csv.close()
+            self._csv = None
 
     def send(self, command: str) -> None:
         if self.ser is None:
@@ -186,20 +185,22 @@ class SerialWorker:
             if enabled and not self.state.csv_logging:
                 log_dir.mkdir(parents=True, exist_ok=True)
                 path = log_dir / f"dryer_{datetime.now():%Y%m%d_%H%M%S}.csv"
-                self._log_fh = path.open("w", encoding="utf-8")
-                self._header_written = [False]
+                self._csv = CsvSession(path)
                 self.state.csv_path = path
                 self.state.csv_logging = True
-                self.state.messages.append(f"CSV -> {path.name}")
+                self.state.messages.append(f"CSV -> {path.name} (on status)")
                 self.send("log on")
             elif not enabled and self.state.csv_logging:
                 self.send("log off")
                 self.state.csv_logging = False
                 self.state.csv_path = None
-                if self._log_fh is not None:
-                    self._log_fh.close()
-                    self._log_fh = None
-                self.state.messages.append("CSV logging off")
+                if self._csv is not None:
+                    rows = self._csv.row_count
+                    self._csv.close()
+                    self._csv = None
+                    self.state.messages.append(f"CSV logging off ({rows} rows)")
+                else:
+                    self.state.messages.append("CSV logging off")
 
     def _note(self, line: str) -> None:
         with self.lock:
@@ -221,14 +222,14 @@ class SerialWorker:
                 continue
 
             if line.startswith("csv,") or line.startswith("csv_hdr,"):
-                if self._log_fh is not None:
-                    write_csv_row(self._log_fh, line, self._header_written)
                 continue
 
             parsed = parse_status(line)
             if parsed:
                 with self.lock:
                     apply_status(self.state, parsed)
+                if self._csv is not None:
+                    self._csv.write_status(parsed)
                 continue
 
             if line.startswith("target="):