What does it mean?
C1805 is a Chassis (ABS, traction control, steering) fault code. It indicates: Mismatched PCM and/or ABS-TC Module .
Symptoms
- ABS and traction control warning lamps on with both systems disabled
- MIL (check engine) may be on as well
- Traction control cannot be switched back on with the dash button
- Speedometer reading slightly wrong if the tyre size or axle ratio data disagrees
- Shift quality poor or inconsistent because the modules are working from different speed data
- The code sets as soon as the ignition is switched on and returns straight after clearing
Likely causes
- ABS/traction module or PCM replaced with a unit built or coded for a different option package
- A module installed without running the programming and configuration procedure
- Used module fitted from a donor vehicle and still carrying the donor's configuration
- Software calibration levels that no longer match after only one of the two modules was updated
- Configuration data corrupted by a failed flash or by a voltage drop during programming
- Axle ratio, tyre size or brake package changed on the vehicle without updating the module data
- Wrong part number supplied because the order did not match the vehicle identification
How to fix it
- Record both module part numbers and calibration levels and compare them with the vehicle's build data
- Read the configuration in each module and compare axle ratio, tyre size, brake and option coding line by line
- Confirm battery voltage is high and stable enough to support programming before starting
- Reprogram or reconfigure whichever module carries the wrong calibration
- If a donor module cannot be reconfigured for this vehicle, fit the correct part number
- Verify the vehicle's actual tyre size and axle ratio match what is coded, especially on modified vehicles
- Clear the codes, run the steering angle and yaw sensor calibrations, then road test to confirm the systems arm