I got a C.E.L. [check engine light] on 02 Mercury grand Marquis while at regular operating temperature on a 30 degree F day. Engine had been running for 2 hours when the wife said the light appeared. I drove the car on two occasions, 70 miles each and the C.E.L. remained on. I went to the parts store and they read the OBD code =P0125 “Insufficient coolant temperature for closed loop control”. I turned off the C.E.L. using their OBD reader. Engine coolant level is topped off [< 1qt added]. I drove the car on two days and the C.E.L. light did not appear. I do not know what the instrument panel temperature gauge reading is on these trips.
On 3rd day, with outside air temperature at 27 degrees F, C.E.L. appears. Engine temperature gauge on the instrument panel is at the very bottom of the range. I assume that the thermostat is stuck open but heater is keeping us comfortable.
My 2002 Ford workshop manual tells me to conduct a ‘thermostat electrical test’.
Remove engine coolant temperature [ECT] sensor harness connector
Attach ECT sensor “T” cable as a jumper between ECT sensor and the power train control module [PTC].
I cannot find the ECT sensor.
I have found the PTC but it has a mass of wires. Perhaps making that connection properly is far beyond my pay-grade. There is no thermostat heater on the car.
The original equipment thermostat is $10. Should I forget about the electrical test and replace the thermostat or am I headed down the wrong diagnosis path?
I would skip the testing and swap out the thermostat. It probably needs one anyway. And like you said, very reasonable on price.
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