DTC P0128: ECT Less Than Thermostat Regulating Temperature: Description
WARNING: This page is about a different car, the 2003 GMC Sierra, 2003 GMC Cab & Chassis Sierra, 2003 Chevrolet Silverado, and 2003 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
An Engine Coolant Temperature (ECT) sensor monitors the temperature of the coolant. The fuel burned since start up is used to determine if the engine has been driven within conditions that would allow the engine coolant to heat up normally to the thermostat regulating temperature. If the coolant temperature does not increase normally, or does not reach regulating temperature of the thermostat, diagnostics that use engine coolant temperature as enabling criteria may not run when expected.
If engine coolant fails to reach a preset target temperature before the calculated amount of fuel is burned, DTC P0128 will set. This Diagnostic Trouble Code (DTC) will only run once per ignition cycle until a Pass, Fail, Disable condition exists.