Clearing fault codes (Mode 04)
Definition
Clearing codes sends a Mode 04 request that erases stored and pending DTCs, freeze frame data, the MIL request and - importantly - resets all readiness monitors to not complete and wipes learned adaptation values in many ECUs (fuel trims, idle, misfire counters); permanent codes are not cleared and only disappear when the monitor passes. It should be done after a repair to verify that the fault does not return, never as a 'fix' or before an emissions test, because an inspector will see all monitors incomplete and most programmes reject the car. Before clearing, read every module, save or photograph codes, freeze frame and mileage, and note current versus stored status, since this history is what lets you reproduce an intermittent fault; after clearing, drive the relevant cycle and re-read pending codes rather than waiting for the lamp. Disconnecting the battery also clears many modules but adds side effects: lost radio codes, window and sunroof one-touch, steering angle sensor zero, throttle adaptation, DPF and transmission learning. Some faults need module-specific reset routines (service interval, battery registration, DPF ash) that Mode 04 does not touch. Related codes: P1000 (Ford: readiness incomplete after clearing), permanent codes P0420/P0171/P0300 that survive clearing, P0601-P0606 if clearing fails due to module faults.