contracts/docs/impl/reports/Z27AG-DECODE-2.md

type, date

type	date
report	2026-10-15

Z27AG CAN: second pass, against OBD-II and GNSS

Sessions 000007-20261015T192938 and 000008-20261015T200202 (car Orin): doors, tailgate, bonnet and wipers (checklist step 10–11), then a short drive with GNSS (3D fix, helix antenna) and OBD-II through the ELM327 on the K-line, all on the Orin's clock. The first time CAN, OBD-II and GNSS were recorded together. About 30 minutes; 1190 OBD-II samples of each fast value.

Confirmed

Signal	Where	Evidence	Registered as
Engine speed	210 bytes 4–5, BE, 1 rpm (pass 1)	equal to OBD-II 0C: median ratio 1.0000, median difference 4 rpm, 1190 samples	engine_rpm (unchanged)
Coolant temperature	449 byte 2, −40 °C	equal to OBD-II 05 within ±1 °C (median 0.0) over 86–97 °C; no other byte tracks it	coolant_temp_c
Vehicle speed	231 bytes 6–7, little-endian, 0.0577 km/h per count	steady cruising (> 15 km/h, GNSS change < 1 km/h over 2 s, 117 fixes): spread 2.6 % against GNSS	vehicle_speed_kph
Four wheel speeds	235 four words, little-endian (pass 1 left the byte order open)	big-endian misses by 28 km/h rms; little-endian fits. Words 0, 1: 0.0578; words 2, 3: 0.0581 km/h per count against GNSS	not yet: which word is which wheel waits for the jack-stands run (fronts turn, rears stay)

Scales are against GNSS, so they include this car's tyres today. OBD-II speed reads 3 % low against GNSS (0.9706): the ECU's figure is no better a reference than the ABS frames. The two word pairs of 235 differ by 0.5 %, the usual size of a front/rear tyre difference.

Doors, tailgate, bonnet, wipers

Thing	Bit	Notes
Driver's door	423.0.4	its own bit (pass 1); also sets any-door
Passenger, rear left, rear right	only 423.2.1 (any door)	no bit of their own
Tailgate	only 423.2.1	with the driver's door open, 423.0.4 cleared on closing it while 423.2.1 stayed set until the tailgate shut
Bonnet	nothing	a mechanical latch with no switch, as expected
Front wiper running	423.3.3	set through "low" and "high"; nothing in a 4 s "interval" window. Medium confidence; also explains pass 1's unexplained 423.3.3 stretch

IMU calibration, checked against GNSS

Level from session 000006 (two still stretches, the ground's 1.9° slope cancelled), forward from 000008 (three stops, one launch; 3512 samples, mean spread 12.7°). The module sits upside down, about 24° off level, turned so its X axis points forward. Calibrated, the marked stretches read:

Stretch	IMU longitudinal (0.25 s mean, peak)	GNSS
Stop, 48 → 0 km/h	−0.35 g	−0.28 g
Stop, 45 → 0	−0.17 g	−0.17 g
Stop, 40 → 0	−0.21 g	−0.18 g
Launch	+0.32 g	+0.36 g

Lateral stays at about 0 in all four: forward is straight ahead. GNSS reports about once a second, which smooths short peaks.

Not found

• Intake air temperature: no byte of 449 tracks OBD-II 0F (29–51 °C here).
• 449 bytes 0, 1, 3 move like temperatures but match nothing on OBD-II.

Notes on the recording

• The Orin froze mid-drive (19:59; hardware watchdog reset at 20:01:47), splitting the drive over two sessions. orin-nano-env-setup has the investigation.
• OBD-II through the ELM gave each fast value about every 2 s (K-line, ~250 ms per request).