C0121Valve Relay Circuit Malfunction
- Generic OBD-II code · Chassis
- Can trigger limp mode
- How the car detects it
- C0121 is absent from the accessible J2012 lists. GM includes it in the chassis code keyword list of bulletin PIP5094 (2013), and AA1Car's ABS code table, which follows GM numbering, names it Valve Relay Circuit Malfunction. This relay passes battery voltage to all solenoid valves of the hydraulic unit, while the controller grounds each valve on its own. A Hella/Lucas patent describes the monitoring: the module checks the relay coil current and the voltage behind the contact, and if either is wrong it drops the relay so the valves fall back to their rest position; a Toyota/Denso patent uses a cut-off relay in the same way. Ordinary braking remains, but ABS and the functions built on it are lost and the warning lamp lights. A sticking contact, a blown valve feed fuse or a poor module ground are typical causes.
- What you may notice
- ABS warning lamp on, often together with traction or stability lamp
- No anti-lock action, wheels can lock under hard braking
- Likely causes
- LikelyBlown fuse or poor battery feed to the valve relay
- PossibleCorroded or loose ground of the brake control module
- PossibleValve relay contact sticking or burnt (relay often inside the module)
- Repair
- Hard · Usually a job for a workshop.Typical cost: €20 – €1,200
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.