Loss of DTC Memory
WARNING: This page is about a different car, the 2004 GMC Sierra, 2004 GMC Cab & Chassis Sierra, 2004 Chevrolet Silverado, and 2004 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
Inspect for loss of DTC memory by disconnecting the engine coolant temperature (ECT) sensor and idling the engine until the MIL turns ON. An ECT DTC should store and remain in memory when turning OFF the ignition for at least 10 seconds. If not, the ECM is malfunctioning.