Disconnect Battery
WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 2003 Pontiac Grand Prix, 2003 Pontiac Bonneville, 2003 Chevrolet Monte Carlo, 2003 Chevrolet Impala, and 2003 Buick Regal. However, it is still accessible from the selected car via links, so may be relevant.
NOTE:
On many OBD-II systems, PCM will retain memory for an extended period of time with battery disconnected. Memory may be retained for several days.
Disconnecting PCM power or battery ground will clear all PCM memory including DTCs, freeze frame, failure records, adaptive learning and system monitors.