Diagnostic Aids

WARNING: This page does not describe the selected car, but rather 11 other vehicles, including the 2002 GMC Yukon XL, 2002 GMC Yukon, 2002 GMC Sierra, 2002 GMC Cab & Chassis Sierra, and 2002 Chevrolet Tahoe. 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 by-pass 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.