What does it mean?
C1256 is a Chassis (ABS, traction control, steering) fault code. It indicates: Brake Booster Pump Motor Relay Ground Circuit Failure . This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms
- Brake and ABS warning lamps on, stability control disabled
- Hard brake pedal with reduced power assist
- Pump motor silent, or cutting in and out especially over rough roads
- Fault appears and clears with vibration, so it may seem intermittent
- Brake system message in the instrument cluster
Likely causes
- Loose, corroded or paint-insulated ground stud serving the booster pump motor relay
- Open or high resistance in the ground wire between the relay socket and chassis ground
- Corrosion inside the ground terminal of the relay socket
- Chafed harness that grounds intermittently against the body or a bracket
- Internal relay failure on the ground side of the coil or contact set
- Salt and moisture damage where the ground path runs through the underhood fuse box
- Control module low-side driver fault (least likely)
How to fix it
- Locate the relay ground point and inspect it for rust, paint under the ring terminal and loose fasteners
- Carry out a voltage drop test on the ground side while the pump is running; it should stay under about 0.1 V
- Wiggle the harness near the relay and the ground stud while watching live data for the fault to toggle
- Measure resistance from the relay socket ground terminal to a clean chassis point
- Clean, re-terminate and torque the ground connection, then retest
- Repair chafed sections of harness and protect them with loom
- Replace the relay if the socket and ground path are proven good, and consider the module only after that