P0128 Diagnostic Test Code (DTC) Details - 2001 Chevy Venture
General Description: P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Detailed Description:
The engine coolant temperature did not reach the correct temperature (at least 176°F for this engine) within the expected time (between 2-30 of start time).

Conditions for this code to set:
If the engine computer detects the calibrated amount of air flow (determined using the IAT sensor) and engine run time have been met, and the coolant temperature has not met the minimum thermostat regulating temperature, the P0128 code will be set.

What to check for:
Thermostat stuck OPEN - This is a common condition if the coolant temperature rises slowly.
Cooling fans stuck ON - Air flows through the radiator too much (fans should cycle ON & OFF, not stay on all the time).
Low Coolant Level - If air is in the cooling system, the temperature sensor may not read high enough.
Bad ECT Sensor or wiring - This can be tested with a multi-meter or scan tool that displays the coolant temperature.
Faulty Intake Air Temperature (IAT) sensor - This sensor is also used for this test and should be checked.

Location of main sensor:
The ECT sensor is located near the thermostat housing, which is near the throttle body.

Notes: The cooling fans are turned ON when certain engine coolant temperature (ECT) codes, such as P0128 are set.
The diagnostic test for this condition will only run once per ignition cycle within the enabling conditions.  The "Service Engine Soon" light will not be turned on unit this test fails twice, but the computer will set the temperature gauge to its minimum setting when the test fails the first time.  

Important Note: Just because a code names a particular sensor or component, it does not mean that sensor or component is bad. Proper diagnostic procedures must always be performed to find the root cause of the problem. 

 

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