U0121Lost Communication With Anti-Lock Brake System (ABS) Control Module A
- Generic OBD-II code · Network
- Can trigger limp mode
- How the car detects it
- U0121 is stored by any listener that stops receiving the ABS/ESC module, so it often shows up in engine, transmission, cluster and gateway at once. GM treats it as Type C without MIL (Chrysler bulletins list it with MIL): the 2019 LTG engine table watches frames $0C1/$0C5 (0.5 s) and the 2025 TCM table waits over 10 s; while it is active, the ECM skips wheel-speed plausibility checks. Ford's 2023 diesel ECU flags the ABS master message after 5 s; Mitsubishi's PHEV-ECU checks it at 10–16.5 V. Field causes: water in the EBCM connector (GM PIT5499A), ground G907A (Jeep), ABS software that locks the car in Park (Chrysler 08-043-26), an ESC restart on the Audi e-tron (2072977/1). GM 16-NA-177 calls a history U0121 in the front camera harmless.
- What you may notice
- ABS, ESC and brake warning lamps on
- Speedometer or gear display erratic, several U codes
- Reduced brake assistance or vehicle stays in Park
- Start-stop and driver assistance unavailable
- Likely causes
- LikelyWater ingress or corroded terminals at the ABS/EBCM connector
- LikelyPoor ground point or supply fuse of the ABS module
- PossibleABS/ESC software that fails to wake up or restarts (update released)
- PossibleChafed or pinched CAN wiring
- RareABS control module internally faulty
- Repair
- Medium · Usually a job for a workshop.Typical cost: €50 – €1,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.