OBD I VEHICLES
Stored codes are removed from memory by removing power to the ECU. Disconnecting the power may also clear the memories used for other solid-state equipment such as the clock and radio. For this reason, always make note of the radio presets before clearing the system. Additionally, some radios contain anti-theft programming; make sure you have the code number before clearing the codes.
While disconnecting the battery will clear the memory, this is not the recommended procedure. The memory should be cleared after the ignition is switched OFF by removing the appropriate fuse for at least 10 seconds. The correct fuses and their locations are:
Codes may also be cleared using the a suitable scan tool, following the tool manufacturer's directions.
OBD II VEHICLES
It is not recommended that the negative battery cable be disconnected to clear DTC's. Doing so will clear all settings from the vehicle's computer system, resulting in lost radio presets, seat memories, anti-theft codes, driveability parameters, etc., and there are better ways of spending your afternoon than resetting all of these.
Control module reset procedures are a very important part of OBD II system diagnostics. This step should be done at the end of any fault code repair and at the end of any driveability repair.
Clearing codes can be performed by either of the methods below:
Removing the negative battery cable may cause other systems in the vehicle to loose their memory. Prior to removing the cable, ensure you have the proper reset codes for radios and alarms.
The MIL may also be de-activated for some codes if the vehicle completes three consecutive trips without a fault detected with vehicle conditions similar to those present during the fault.