Network, generic

U0121 Lost Communication With Anti-Lock Brake System (ABS) Control Module A

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

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.

Likely causes

Water ingress or corroded terminals at the ABS/EBCM connector
Poor ground point or supply fuse of the ABS module
ABS/ESC software that fails to wake up or restarts (update released)
Chafed or pinched CAN wiring
ABS control module internally faulty

Symptoms

  • 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

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€50–1,500
Time0.5–4 h

Typical range across vehicles; the real cost depends on what the testing finds.

What to check first

  1. Check ABS fuses and grounds
  2. Check network wiring
  3. Have braking safety checked

Parts involved

abs control modulecan bus wiring

Related codes

U0415U0315U0073U0122U0129

References: gsi.ext.gm.com, gsi.ext.gm.com, fordservicecontent.com, mitsubishitechinfo.com.