What does it mean?
C1704 is a Chassis (ABS, traction control, steering) fault code. It indicates: EPS ECU Fail-Safe Relay Fail .
Symptoms
- Power steering / EPS warning lamp on
- Steering suddenly becomes very heavy as the assist cuts out, often intermittently at first
- Assist returns after an ignition cycle and then drops again
- An audible click from the column area at the moment assist is lost
- Much higher effort at parking speeds while remaining normal at highway speed
Likely causes
- Fail-safe relay contacts inside the EPS ECU welded closed or burned open
- Weak battery or a charging system fault letting supply voltage collapse and drop the relay out
- High resistance at the heavy power feed connector, its fuse or the fusible link
- Poor ground at the steering control unit mounting point
- Shorted assist motor winding drawing excess current and forcing the relay open
- Water ingress into a column-mounted EPS ECU corroding the board
- Failed internal driver circuit that energises the relay coil
How to fix it
- Test the battery and charging system before anything else; this circuit is very voltage sensitive
- Inspect and load-test the EPS power feed fuse or fusible link and its connector
- Perform a voltage drop test on the EPS supply and ground while an assistant turns the wheel
- Watch EPS supply voltage and motor current in live data during a lock-to-lock turn
- Wiggle-test the harness and connector to reproduce the dropout
- Open and inspect the ECU area for moisture, corrosion or heat damage
- The relay is internal and not serviceable: if supply, ground and motor are good, replace the EPS ECU/column unit and run the steering angle calibration