Code P0128 on the 2024 Chevrolet Equinox indicates the engine coolant temperature is not reaching the expected operating range within the specified time. While the code description mentions 'below thermostat regulating temperature,' it typically indicates the thermostat is stuck open, not closed - resulting in overcooling.
Understanding P0128
The ECM monitors how quickly the engine reaches operating temperature after a cold start. If the coolant temperature doesn't reach approximately 160-190°F within a calculated time based on ambient temperature and driving conditions, P0128 sets. This indicates the thermostat isn't restricting coolant flow to allow warmup.
Overcooling Effects
An engine running too cool experiences reduced fuel economy (ECM keeps richening fuel for warmup), increased emissions, poor heater output, and potential increased wear from running cold. The ECM cannot enter closed-loop fuel control until proper temperature is reached.
Common Causes
A thermostat stuck open is the most common cause - the valve fails to close, allowing constant coolant circulation through the radiator. A missing thermostat (from improper repair) causes the same symptom. The coolant temperature sensor could be reading incorrectly low, or in cold climates, the cooling system may actually be overcooling without insulation.
Diagnostic Approach
Monitor coolant temperature during warmup - it should rise steadily and reach operating temp within minutes of normal driving. Feel the upper radiator hose - it should stay cool until the thermostat opens at operating temperature. If it gets warm immediately, the thermostat is stuck open.