OBD-II code · Network
U01B4 Lost Communication With Brake System Control Module B
ECU lost communication with brake system control module B (secondary brake controller). A communication loss with this module breaks any function relying on its data — diagnostic tools may show 'no response' and dependent modules can fall into limp mode.
Is it safe to drive with U01B4?
The car may go into limp mode (reduced power). Get it diagnosed before a long trip.
Common causes
- Most likelyModule unpowered
- Less likelyGateway module routing CAN messages incorrectly
- Less likelyCAN terminating resistor (120 Ω) drifted out of spec
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
Repair at a glance
- Difficulty
- Hard
- DIY friendly
- Best left to a mechanic
- Typical labour
- 1–8 h
- Emissions test
- Usually unaffected
- After the fix
- A drive cycle is needed before the light stays off
- Parts involved
- Brake System Module B, Can Bus
Related codes
- U01B1Lost Communication With Particulate Matter Sensor B
- U01B2Lost Communication With Planetary Gear Set Control Module
- U01B3Lost Communication With Brake Blending Actuator
- U01B5Lost Communication With Catalyst Heater Control Module
- U01B6Lost Communication With Fuel Heater Control Module
- U01B7Lost Communication With Switch Input Module A
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.