Diagnostic Aids
WARNING: This page is about a different car, the 2002 Pontiac Sunfire, 2002 Pontiac Grand Am, 2002 Oldsmobile Alero, and 2002 Chevrolet Cavalier. 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 IAT sensor may cause DTC to set. IAT sensor should read near ambient temperature with engine running.