Conditions for Setting the DTC: P0128

WARNING: This page is about a different car, the 2008 Cadillac CTS. However, it is still accessible from the selected car via links, so may be relevant.

The ECM detects that the actual engine coolant temperature is 11°C (20°F) less than the calculated thermostat regulating temperature of 89°C. (192°F), for greater than 4 seconds or for a cumulative of 30 seconds.
OR
The ECM detects that the actual engine coolant temperature is 10°C (18°F) less than the calculated ECT for greater than 1 second or a cumulative of 10 seconds, or 4 seconds or a cumulative of 30 seconds for LLT.