C0121: Valve Relay Circuit (GM)
The ABS module has detected a fault in the circuit that powers the ABS valves.
High risk
Have it checked soon. ABS and stability control may be off.
Short answer · Can I keep driving with C0121?
Normal braking usually remains, but ABS may not work. Drive carefully and have it checked.
What does C0121 mean?
C0121 refers to the relay or driver that supplies the ABS solenoid valves. Low voltage, a blown fuse or an internal fault can set it.
Note: this page uses the definition used by General Motors. Other makes may use the code differently; check the description your scanner shows.
What the car does when the code is set
The ABS and stability control warning lights come on, and the systems are switched off. Normal braking remains, but the wheels can lock under hard braking.
How the system works
The ABS hydraulic unit contains a pump motor and valves that control brake pressure at each wheel. The control module switches them through relays or drivers and monitors the circuits.
Common symptoms
Possible causes
Listed without ranking. The cause on your car depends on make, engine and mileage — see the maker notes below.
- Blown fuse or failed relayThe ABS pump and valves have high-current fuses.
- Poor ground at the ABS unitCorrosion at the ground point.
- Connector corrosionThe ABS unit sits in the engine bay and is exposed to moisture.
- Low battery voltageThe pump needs high current.
- Failed ABS hydraulic unit or control moduleInternal faults in the pump motor or valve relay.
How it is diagnosed
- Check the fuses for the ABS unitUse the fuse diagram; ABS pump fuses are often large cartridge fuses.
- Check battery voltageCharge or test the battery.
- Inspect connector and groundLook for corrosion and loose terminals.
- Have the unit testedA workshop can command the pump and valves with a scan tool.Brake hydraulic work must be done by trained technicians.
Questions about C0121
Can a weak battery set C0121?
Yes. Check battery voltage before replacing parts.
Do the brakes still work?
Normal braking usually remains, but without ABS the wheels can lock under hard braking. If the brake warning light is also on, stop and have the car checked.