Conditions for Setting the DTC
WARNING: This page is about a different car, the 2007 Pontiac G5 Pursuit, 2007 Pontiac G5, and 2007 Chevrolet Cobalt. However, it is still accessible from the selected car via links, so may be relevant.
DTC P0128
The ECM detects that the minimum ECT of 55°C (131°F) has not been met with a start-up IAT of less than 10°C (50°F).
OR
The ECM detects that the minimum ECT of 80°C (176°F) has not been met with a start-up IAT of greater than 10°C (50°F).