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I have a 2001 Lincoln Navigator that intermittently throws a P0125. From my research the most likely cause is Thermostat or Engine Temp Sensor. Is there a procedure to determine if the Thermostat is stuck open ?

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Code P0125: Insufficient coolant temperature for closed loop operation.

The code sets under these conditions:
Engine run-time at road load over 6 minutes at which time the PCM detected the Engine Coolant Temperature (ECT) sensor signal did not indicate the required engine temperature value to enter Closed Loop within a specified amount of time. The amount of time is calculated from the point at which the engine is started. And it depends upon the engine coolant temperature sensor temperature at startup. This formula is not published.

Causes and tests;

Common: Check the operation of the thermostat (it may be stuck open). Test for an open thermostat by monitoring the radiator inlet tank temperature during warm up from a cold start. An open thermostat will allow warm coolant into the radiator very soon during warm-up. The thermostat should not allow coolant to flow into the radiator until the engine reaches at least 175 deg F. Since this is intermittent and therefore may not be seen as failed during testing; I would replace the thermostat as it is the most common cause of this code. Replace the coolant if the time in service has been met or it looks unusual or if the freeze temperature measures above -30 Deg F

Uncommon: Check the coolant mixture for an incorrect mixture, and the coolant level

Rare: Engine coolant temperature sensor sensor has failed.

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