Checklist v4: step 31, each wheel by hand on stands; DECODE-3: 449 bytes 0, 1, 3 as climate-control sensor candidates

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DDwfd6Ki8UfG4hK6W5YpJz

docs/impl/reports/Z27AG-DECODE-3.md

@@ -64,6 +64,32 @@ Jack stands (checklist 23–26) would still confirm this, but are no longer need
   Scale unknown: full is unlikely to be 100 or 255. A refuel gives two points (before and after,
   parked level, with the litres put in). Not registered until then.
 
+- **`449` bytes 0, 1, 3: climate-control temperatures, most likely raw sensor readings.** Pass 2 looked
+  for a byte that equals OBD-II intake air temperature less 40 and found none. Read the other way,
+  bytes 1 and 3 run *against* it (correlation −0.96 and −0.98 in `000007`), the way a thermistor's raw
+  reading falls as it warms. They follow it over tens of minutes, not second to second, so they are
+  other sensors that warm and cool with the air around the engine and car:
+  - **Byte 3: the A/C evaporator.** At each first switch-on of the compressor (`ac_compressor`) it climbs
+    from 115–145 to about 185 within 30–60 s, then stays there while the compressor cycles on and off
+    every 10–20 s. That is the evaporator being held just above freezing, the job its sensor exists
+    for. It keeps climbing a few counts after each switch-off, as an evaporator does while the last
+    refrigerant boils off. With the A/C off it drifts with the air (the `000007` correlation above).
+    Higher count, colder.
+  - **Byte 1: outside air, probably.** Slow; 89–91 on the afternoon of the 13th, 99–117 in the evening
+    of the 15th (higher count, colder, as byte 3). It fell from 114 to 99 over 30 minutes parked and
+    idling, and rose from 112 to 117 over 8 minutes of driving. A sensor behind the grille does this:
+    radiator heat soaks it while standing, and the airflow cools it once moving. That is why climate
+    controls filter it.
+  - **Byte 0: unclear.** 119–134, slow, unmoved by the engine starting or stopping (so not the
+    charging voltage), and its sign against the other two is not consistent. A cabin sensor is one
+    guess.
+
+  An auto climate control has these three: evaporator, outside air, cabin. The way to scale them is
+  an overnight soak: a cold start after the car has stood all night gives one reading of each with
+  everything at the outside temperature (OBD-II intake and coolant say what that is). With a
+  thermometer and the A/C steps (12, 17), it gives a second. A mid-session frame with byte 6 = 24 instead
+  of 51 (end of `000009`, engine stopping) has every byte halved or changed; left out of the above.
+
 ## Notes on the recording
 
 - `000012` is a 25-second parked session in between; `000013` ends with the PEAK unplugged.