Diagnostic Aids

WARNING: This page does not describe the selected car, but rather 13 other vehicles, including the 2002 GMC Yukon XL, 2002 GMC Sierra, 2002 GMC Savana Special, 2002 GMC Savana Camper Special, and 2002 GMC Savana. However, it is still accessible from the selected car via links, so may be relevant.

A DTC P0128 is designed to detect a faulty thermostat.
Excessive coolant flow through the bypass hose to the throttle body may cause this DTC to set.
A skewed ECT sensor may cause a P0128 to set.
A skewed IAT sensor may cause this DTC to set.