Join Date: Oct 2011
Location: Magnetic Hill, New-Brunswick
Nothing is instantaneous, the ECM controls all engine issues the way it was programmed. It's the ECM that sends the reading to the DIC, it's the ECM that sends the check engine light command to the instrument panel and it's the ECM that turns on the fan before or after the code comes up. If your thermostat is 11 years old it is long overdue and has lasted longer than most.
Here is what the GM service data base says.
An engine coolant temperature (ECT) sensor monitors the temperature of the coolant. This input is used by the powertrain control module (PCM) for engine control and as an enabling criteria for some diagnostics.
The air flow coming into the engine is accumulated and used to determine if the vehicle has been driven within the 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 the regulating temperature of the thermostat, the diagnostics that use ECT as enabling criteria may not run when expected.
If the PCM detects the calibrated amount of air flow and engine run time have been met, and the ECT has not met the minimum thermostat regulating temperature, DTC P0128 sets.
This diagnostic procedure supports the following DTC:
DTC P0128 Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature
Conditions for Running the DTC
DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0502, P0503 are not set.
The startup ECT is less than 65°C (149°F).
The intake air temperature (IAT) is between -7 and +55°C (+19 and +131°F).
The engine run time is between 30 seconds and 30 minutes.
The vehicle is driven more than 0.8 kilometers (0.5 miles) at more than 8 km/h (5 mph).
The mass air flow (MAF) is between 2-30 g/s, with the average more than 1 g/s.
This DTC runs once per ignition cycle within the enabling conditions.
Conditions for Setting the DTC
The PCM detects that:
The calibrated amount of engine run time has been met
The calibrated amount of engine air flow has been met
The calibrated vehicle speed and distance have been met.
The calibrated ECT of 70°C (158°F) has not been met
Never drive faster than your guardian angel can fly