Chassis, generic

C0121 Valve Relay Circuit Malfunction

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

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.

Likely causes

Blown fuse or poor battery feed to the valve relay
Corroded or loose ground of the brake control module
Valve relay contact sticking or burnt (relay often inside the module)

Symptoms

  • ABS warning lamp on, often together with traction or stability lamp
  • No anti-lock action, wheels can lock under hard braking

Repair estimate

DifficultyHardWorkshop job
Parts and labour€20–1,200
Time0.5–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

abs valve relayabs control moduleabs hydraulic unit

Related codes

C0225C0220C053BC0110

References: static.nhtsa.gov, aa1car.com, patents.google.com, patents.google.com.