C0121
Valve Relay Circuit Malfunction
The ABS control module has detected a fault in the valve relay circuit, which supplies power to the ABS solenoid valves. With this fault, the module cannot energize the valves and ABS is disabled.
- Check engine light: Not always
- Emissions related: No
- Repair difficulty: hard
Symptoms
- ABS warning light on
- ESC/stability control deactivated
- Possible reduced brake assist
- Speedometer may behave erratically
Common causes
Ordered by how often each turns out to be the cause.
- Valve relay failedCommon
- Internal ABS module fault (relay integrated)Possible
- Power supply wiring or fuse faultLess likely
Parts involved
- ABS Valve Relay
- ABS Control Module
- ABS Hydraulic Unit
C0121 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.