Error code

OBD2 U0121

OBD2U0121
Diagnostic Trouble Code· Standard OBD-II fault code read from the vehicle's onboard diagnostics.

What it means

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

Severity

highCan drive, but with limited functionality and potential safety risks.

Summary

Code U0121 indicates a loss of communication between the anti-lock brake system (ABS) control module and the Powertrain Control Module (PCM). This issue can affect vehicle safety systems like Traction and Stability Control.

Symptoms

  • Vehicle may enter limp mode, limiting throttle control or preventing transmission shifts.
  • Vehicle might not start.
  • Instrument cluster could fail.

Common Causes

  • Damaged wiring and connectors
  • Defective ABS control module
  • Voltage spikes from incorrect jump starting procedures
  • Low battery voltages or defective batteries
  • Failed PCM (rare)

Diagnosis

  1. Record all fault codes and freeze frame data.
  2. Locate the ABS control module and wiring harness.
  3. Inspect for damaged wiring and connectors.
  4. Replace any faulty wiring or connectors.
  5. Test resistance, ground, reference voltage, and continuity on associated wiring.

Fixes

  • Replace the defective ABS control module if necessary.

FAQ

Can I drive with U0121?

You can drive, but it's risky due to potential safety issues and limited vehicle functionality.

What causes U0121?

Common causes include damaged wiring/connectors, defective ABS control module, voltage spikes, low battery voltages, or a failed PCM (rare).

View source

Not affiliated with OBD2. Answer summarized from the manufacturer's manual linked above; consult that source for the authoritative procedure.

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