Diagnostic Aids

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.

DTC P0128 is designed to detect a faulty thermostat.
A skewed low ECT sensor can cause a DTC P0128 to set.
If an intermittent condition is suspected refer to Intermittent Conditions .
Using Freeze Frame/Failure Records may aid in determining the conditions under which the failure occurred.