What does it mean?
C1166 is a Chassis (ABS, traction control, steering) fault code. It indicates: ABS Hydraulic Valve Power Relay Circuit Open . 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
- ABS lamp on from the moment the ignition is switched on, with the traction and stability lamps alongside it
- No self-test click or hum from the ABS unit at key-on, because the valve block never energises
- ABS does not work at all: no pedal pulsing, and the wheels can lock during emergency braking
- Normal service braking is unaffected, so the fault is easy to miss until a hard stop is needed
- Hill-start assist, brake force distribution and any off-road brake modes are unavailable
Likely causes
- Blown high current fuse feeding the ABS hydraulic unit
- Relay coil open internally, or the relay not fully seated in its socket after previous work
- Broken wire inside the loom between the fuse box, the relay and the hydraulic unit, often at a flex point
- Spread, burnt or backed-out terminals in the relay socket or the ABS module connector
- Corrosion on the ABS module power pins after water has entered the connector
- Open ground strap or a loose ground stud for the ABS hydraulic unit
- Failed internal track or connector solder joint inside the ABS control module
How to fix it
- Test the ABS fuses with a meter, high current fuses included, and check the relay is properly pushed home in its socket
- Read all chassis codes; a full set of ABS valve and relay codes together points to one lost power feed
- Swap the relay with a known good identical unit if the fuse box has one, and see whether the behaviour changes
- Measure voltage at the relay socket power and control terminals with the ignition on to see which side of the circuit is dead
- Check continuity of the feed wire from the fuse box through to the ABS module connector, and check the module ground stud for tightness and corrosion
- Inspect the ABS module connector for water, green corrosion and terminals that have backed out of the housing
- Repair the open wiring or terminal, and only then consider ABS module repair or replacement with coding