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Q: p0117 code and engine heating to 75% of temp gauge (but no further)

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I got a p0117 code regarding the ect sensor and the engine being too hot. I just replaced both the temp gauge and the ect sensor (the cooling fan wasn' t turning on). The car stays at 50% of the temp gauge when idling, but when driving it climbs to about 75% of the gauge, then when idling again is meanders back down to the 50% mark. I just replaced the coolant, the radiator cap is in good shape, and the thermostat looks ok (I' ll talk about this later).

My question is what the heck is going on here. Am I naive or could the thermostat not be functioning properly and so it is circulating some, but not all, of the coolant? Is the ECT sensor bad (the replacement)?

The car is not running rough or getting poor gas mileage. It is not smoking or showing milky oil in the dipstick. I do not think it is the PCM or the head gasket (I' m hoping at least) because of what I described at the beginning of this paragraph. I am at a loss as to what it can be at this point, maybe it is the ECT sens

My car has 135000 miles.
My car has an automatic transmission.

A: You will have to scan the computer when the...

You will have to scan the computer when the engine is cold and look at the data to compare the ECT and the IAT. This is the inlet air temperature sensor and the engine coolant temperature sensor. Both sensors will be very close to each other when the engine is cold and not running, but with the key in. If both sensors are within a few degrees of each other, and they are the same as the temperature outside, then the sensors are okay.

If you see one that is off from the other and not close to the outside air temperature, then it is not accurate and should be replaced. The cooling fans use the coolant temperature sensor data for turning on and off the cooling fans, and will turn on the fans at around 227 degrees and turn them off at around 185 degrees. This would account for the fans going on and off and the temperature gauge going up and down.

You should check the radiator for clogged fins or cold and hot spots in the fins indicating a radiator that is not flowing good internally. This would cool the engine at idle but will run hot at higher speeds. This could indicate that the radiator may be clogged.

To pinpoint the issue, a technician from YourMechanic can come to your home or office to diagnose the overheating problem and let you know what needs to be done.

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