Diagnostic Aids

Check for the following conditions:

• A faulty thermostat that stays open slightly can cause DTC P0128 to set in cold weather when the vehicle is started and allowed to sit while warming up. Replace any suspect thermostat. Refer to Thermostat Replacement in Engine Cooling.
• An incorrect thermostat can cause the engine to warm up slowly. Replace any thermostat that does not meet OEM specifications. Refer to Thermostat Replacement in Engine Cooling.
• Use the Temperature vs. Resistance table in order to evaluate the possibility of a shifted ECT sensor. A shifted sensor could result in DTC P0128. Refer to Temperature vs. Resistance .
• Check for the proper operation of the engine cooling system, including the correct coolant level.
• Check for a faulty electrical connection to the PCM.

If DTC P0128 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC was first set.