U0121: Lost Communication With Anti-Lock Brake System (ABS) Control Module
Other modules have stopped receiving messages from the ABS control module, so anti-lock braking and stability control may not work.
What U0121 means
The ABS module controls anti-lock brakes and usually traction and stability control. It also shares wheel speed data that many cars use for the speedometer, transmission and other systems. U0121 means other modules have stopped receiving messages from the ABS module over the network. When that happens, the ABS and stability control are usually disabled, so the car brakes like an older vehicle without ABS. KBB advises against driving because ABS reduces the chance of an accident in emergency braking. Common causes include a blown fuse, a bad ground, a corroded connector and network wiring damage. The ABS module lives near the brake master cylinder or wheel well, where water and road salt can reach it, so corrosion is common. Internal module failures happen on some models too. Shops check fuses and power, test grounds, inspect the connector and measure the network wiring. If the module has power and ground but still will not communicate, it is repaired or replaced.
Common symptoms
- ABS warning light on
- Traction or stability control light on
- Check engine light on
- Speedometer not working on some vehicles
- Brake pedal may feel different in hard stops
Most common causes
- Blown ABS fuse or power feed problem
- Bad ground at the ABS module
- Corroded ABS module connector
- Damaged CAN bus wiring
- Failed ABS control module
- Weak battery or low voltage
What to check yourself first
Check the ABS fuses, which are often high current fuses in the under hood fuse box. Inspect the ABS module connector, usually near the brake master cylinder, for corrosion. Have the battery tested.
Typical fixes and what they cost
| Fix | Typical cost | Note |
|---|---|---|
| Network (CAN bus) electrical diagnosis | $122-$179 | Network faults are usually found by checking power, ground and the two CAN bus wires at each module. |
| Replace ABS control module | $1,181-$1,278 | Some ABS modules can be repaired by specialty rebuilders for less. |
| Replace battery | $456-$493 | A weak battery or low voltage is a very common cause of communication codes. |
| Replace wheel speed sensor | $234-$310 | If wheel speed sensor faults are also stored and affecting the module. |
Typical US price ranges for parts and labor, not quotes. Your shop should confirm the cause with a diagnostic before replacing parts.
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Related codes
- U0100 Lost Communication With ECM/PCM "A"
- U0140 Lost Communication With Body Control Module
- U0073 Control Module Communication Bus "A" Off
- P0500 Vehicle Speed Sensor "A"
- U0155 Lost Communication With Instrument Panel Cluster (IPC) Control Module