This question is in regards XXXXX XXXXX 98 Ford Escort ZX2. Sorry for the lengthy description, hopefully it helps narrow down the cause of the problem. The coolant
temperature gauge is always reading low and the Service Engine
Soon light is on. The code is P0125. AutoZone computer check says 1.) Check coolant level, which I have checked and the coolant tank is between the hot and cold lines. 2.) Thermostat
defective, which we have replaced 3 different times in 5 years for this issue but has never resolved the issue. 3.) ECT (engine coolant temperature) sensor defective, which we replaced a few months ago but the problem remains. There are 2 sensors around the thermostat housing, one on top which we were told sends a signal to the computer, and one on the bottom which sends a signal to the gauge. We replaced the one on the top because we figured that would be the one to trigger the check engine light if not working properly, but it did not fix the problem. We purchased these parts from a Ford dealer, so we figure they are the correct parts. We did NOT replace the sensor on the bottom. This problem seems to only happen when it starts getting cold outside (50 degrees or less). Took car to a mechanic who measured engine temperatures with infrared gun and told me the engine warms up to the actual temperature it should according to his book, but the gauge never moved much above the cold mark. I get plenty of heat from the blower even in the dead of Wisconsin winters. When car is first started, the needle is buried well below the C and when driving on freeway or in city, the gauge rises to the C but doesn't go above it. But if I turn off the engine when it's fully warmed up for ~15 minutes and then start it up, it goes to the halfway point between the C and H, but as soon as I start driving it drops back down to the C again. This tells me bad thermostat, but we've replaced it 3 times with the same result, so I have a hard time believing that the thermostat is the actual problem. After car has been sitting awhile with the engine off, the temp gauge floats to about halfway between the C and H, which I've heard is normal. When I turn the key to the ON position (without starting the engine), the instrument panel is activated and the coolant temp gauge moves to the cold mark, but when I start the engine, it dips way below the C, not sure why. Gas mileage
is noticably lower than it used to be before engine light came on, and I don't think it is the cold weather that is causing that. Engine doesn't sound any different than it always has. If the sensor that sends a signal to the gauge is bad, would it register a code with the computer? Chilton manual wiring diagrams make it look like that sensor only sends a signal to gauge and not to the computer, so if that sensor was bad, it wouldn't seem like it'd register on the computer? Could it be a bad gauge? Since it still rises up to the halfway point when the weather is warm, it seems like the gauge is still good. Problem with the wiring maybe? I apologize again for so much info, I'm just trying to provide as much related info as possible to help narrow it down. Thanks.