Topic
ABS, airbag and network codes (C, B, U)
C codes cover the chassis, B codes the body and airbags, and U codes the network between modules. Most need a scan tool that can talk to the ABS, airbag and body modules on your make.
- Lamps
- ABS, airbag (SRS), traction control
- Code format
- C, B or U + 4 characters
Chassis codes (C) come from systems such as ABS, traction and stability control and electric power steering. A wheel speed sensor fault is a typical example. Body codes (B) include the airbag or supplemental restraint system, seat belt pretensioners, climate control and lighting.
Network codes (U) mean one module lost communication with another, for example U0100, lost communication with the engine control module. The cause is often power, ground or wiring to a module rather than the module itself, and a low battery voltage can set several U codes at once.
Airbag systems need care. Disconnect the battery and wait the time the service manual specifies before unplugging any airbag component, and never probe airbag circuits with a test light. An airbag light that stays on means the system may not deploy in a crash.
Common mistakes
- Expecting a basic engine code reader to show ABS or airbag codes.
- Replacing a module for a U code before checking battery voltage, grounds and connectors.
- Working on airbag wiring with the battery connected.
Pages on this topic
- U0101 code: Lost communication with TCMU0101 means a module stopped hearing from the transmission control module. Learn the common causes, how to diagnose the network, and if it is safe to drive.Fix
- U0100 code: Lost communication with ECM/PCM AU0100 means a module lost contact with the engine computer on the vehicle network. Causes, from a weak battery to CAN wiring faults, and how to diagnose it.Fix
- U0001 code: High speed CAN communication busU0001 means a fault on the high speed CAN network that lets modules talk. Learn the symptoms, common wiring and module causes, and how to diagnose it safely.Fix
- Airbag light on: what it means and what not to doAirbag light on? The SRS system has found a fault and the airbags may not deploy. Common causes, how to read SRS codes safely, and why repairs belong with a pro.Fix
- ABS light on: common causes and how to read the codeABS light on? The common causes, from wheel speed sensors to low brake fluid, whether it is safe to drive, and how to read ABS codes with the right scanner.Fix