P0128 Thermostat Rationality

P0128 Thermostat Rationality - With the engine running, start up coolant temperature less than 65°C (149°F), ambient temperature between -8°C (17.6°F) and 50°C (122°F), avarage vehicle speed greater than 16 km/h (10 mph) until coolant temperature reaches 85°C, engine speed does not go above 5400 rpm for more than 32 seconds, the pass/fail determination point is reached when the coolant temp model reaches 88°C (190.4°F). Set conditions, the predicted coolant temperature reaches the target threshold before the actual coolant temperature and the PCM detects that the actual engine coolant temperature is too far below the what is acceptable for a good thermostat. MIL light will illuminated.

P0128 Thermostat Rationality

Possible causes:
  • Low coolant level.
  • Cooling system issues.
  • Thermostat Operation.

Diagnose code P0128 Thermostat Rationality

Step 1. Low coolant level
If any ECT or AAT DTC are present, diagnose and repair those DTC before continuing with this test procedure.

Warning : Never open the cooling system when the engine is hot. The system is under pressure. Failure to follow these instructions can result in personal injury including extreme burns, scalding, or fatal injury. Allow the engine to cool before opening the cooling system.

Check the cooling system to make sure that the coolant is in good condition and at the proper level.

Is the coolant level and condition OK?

Yes, go to step 2.
No, inspect the vehicle for a coolant leak and add the necessary amount of coolant.

Step 2. Check the cooling system operation
If engine coolant temperature is above 65°C (149°F), allow the engine to cool until it is below 65°C (149°F). Start the engine, check for proper operation of the cooling system while the engine warms up toward operating temperature. Be certain to check the Radiator cap, flow through the Radiator and anything that can cause a lack of system pressure or lack of coolant flow in the system.

Were any issues found with the cooling system?

Yes, perform the appropriate repair in accordance with the service information.
No, repalce the Thermostat.

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