C1805: Mismatched PCM and/or ABS-TC Module

C1805 is a chassis code that flags a mismatch between the powertrain control module (PCM) and the ABS traction control module. These two modules share data constantly, and they are programmed to recognise one another through vehicle identification and calibration data. When one module detects that the other does not match the expected configuration, whether due to incompatible part numbers, incorrect programming, or a module replacement that was not properly configured, C1805 is set. ABS and traction control functions may be disabled as a result.

The most common scenario is a module replacement where the new unit was not programmed to match the vehicle's existing configuration, or a PCM update that introduced a version incompatibility. It can also appear after a battery reset in some vehicles if the modules re-sync incorrectly. Physical faults are less likely here since this is fundamentally a calibration and communication issue rather than a wiring fault. Confirm which module was recently replaced or reprogrammed, and verify that both modules are carrying the correct software and calibration for the vehicle.

This is a medium-severity fault but it is not one you can resolve with a basic code clear. Correct resolution requires a workshop with manufacturer or professional-grade scan tools capable of programming and verifying module configurations. Driving with this code active means you are likely operating without full ABS or traction control, which increases risk in emergency braking or slippery conditions, so book it in soon.