Having U1208 / C1391 coming randomly, and adding P12B4 and P0490.
I've asked ChatGPT to summarize what the cause was, first of all, (for those who don't want to vaste any time reding everything I've checked and tested and replaced.
- Spoiler: show
Always start simple – Wheel speed sensorsRead live wheel-speed data at low speed (5–10 km/h).
→ All four wheels should rise evenly and display identical values.
Measure resistance on each sensor (rear = 2-pin VR type / front = 3-pin Hall on some models).
Normal range: 500 – 1600 Ω
∞ Ω = open circuit · ≈ 0 Ω = short circuit
Measure AC voltage while turning the wheel: typically 30–300 mV AC.
Check sensor air gap and tone ring for rust, metal debris, or moisture.
Check power and groundPin 1 = +12 V / +5 V (depending on sensor type)
Pin 2 = ground
Voltage drop pin 1–2 to chassis should be ≤ 0.05 V under load.
Inspect all ground points (e.g. MC12 in engine bay).
Verify CAN networkBetween OBD pins 6 and 14 → ≈ 60 Ω
With ignition ON: CAN H ↔ GND ≈ 2.5 V, CAN L ↔ GND ≈ 2.5 V.
Continuity OBD ↔ ABS/ECU pins < 1 Ω.
Confirm ABS power supplyFuses F7/F8 → ABS +12 V feed
Ground MC12 → ABS pin 13
11–13 V with ignition ON.
Software diagnosticsIn Diagbox → Global Test → ABS/ESP → ECU Status
If ABS module disappears from the list → communication dropout.
Run Pump Test in Diagbox → ensure voltage never drops below 10 V.
When all above checks passStill faulty? → Suspect intermittent ESP81 module.
Reappears after rain or washing? → Inspect sensor connectors for corrosion.
Only after verifying sensors and wiring should you consider module repair or replacement.
Common real-world causes[Symptom Likely cause Action
U1208 + C1391 Wheel-speed sensor short/open Replace sensor / clean connector
U1208 only ABS module dropout Check power / ground / module integrity
C1391 only Engine-torque message invalid Inspect CAN communication
ESP light, no codes Temporary CAN noise Usually moisture or poor contact at sensor
Tip:Always begin with wheel-speed sensors – in roughly 8 out of 10 PSA cases, even a “communication” or “torque” code originates from a faulty or wet sensor.
My story:
I've started to use ChatGPT to discuss issues...
Started with Diagbox and checked the EGR, which seemed to be ok.
Continued with CAN bus (had to buy a pocket oscilloscope), but ended measuring and Ohming the CAN bus with the ESP/ABS module (60Ω with ABS connected and 120Ω with ABS disconnected).
Voltage test of CAN bus
Checked the contiunuity of the cables for CAN bus, +12V and Ground with a continuity tester.
Gaspedal and torque correlation tested in Diagbox
Checking all connectors on ECU, ESP/ABS, fusebox etc. for corrosion
OK, everything seemed ok, it have to be the electronics in the ESP/ABS unit. Spent €200.00 on a used one.
Same error on test driving.
I tried to monitor wheel speeds on Diagbox while driving, but it lot connection to the ECU all the time.
The last reading I had, was 12km/h on all wheels, except on the left rear wheel, which was 11km/h.
Not much of a difference, but a difference...
And: What would be the simplest thing to test, if I hadn't climbed down the rabbit hole and started to use ChatGPT...?
I would have measured the ABS sensors (Ohming).
Typical 1100Ω one way and 550Ω measuring the other way (changing polarity on my multi meter)...
What did I find on left rear sensor?
550Ω one way and 650MΩ (almost open)
I replaced the sensor, "very" simple on a 2011 C4: Brake calipper off, brake disc with bearing off (36mm center nut) and THEN, the access to the ABS sensor...
1. test trip was successfull. Why didn't I check the ABS sensors first?
