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Shawn, Auto Service Technician
Category: Ford
Satisfied Customers: 3756
Experience:  16 years experience Ford,Nissan and Honda dealership in all areas
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98 Ford Escort ZX2: temperature gauge..the Service Engine Soon light

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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.

Hi megro,


Thanks for using Just Answer, if the cooling system has air pocket in the system, it will also give the PCM (Powertrain Control Module) a false reading, and therefore throw a code, also when using teflon tape on threads, too much and you can also get a false reading. Try hoisting the front of the car so the radiator is the highest point on the engine, and try to burp the system while the car is running and the radiator cap is open, be sure to maintain coolant level.


The sending unit only sends a signal to the gauge, and the ECT sends a signal to the PCM.




Engine Coolant Temperature (ECT) Sensor


The engine coolant temperature sensor signal is used by the powertrain control module to modify ignition timing and the air-to-fuel ratio.








    • Insufficient warm up time.
    • Leaky or stuck open thermostat.
    • Low engine coolant level.
  • Diagnostic Trouble Code (DTC) P0125 indicates the ECT sensor has not achieved the required temperature level to enter closed loop operating conditions within a specified amount of time after starting engine. This DTC will light the MIL.

    Possible causes:

  • Check engine coolant level.

Is the engine coolant level fill correct?

Yes No
GO to the Powertrain/Engine Group, Engine Cooling Section in the Workshop Manual for further diagnostics. FILL engine coolant to proper level. COMPLETE PCM Reset to clear DTCs.








Possible Sources

  • Incorrect Temperature Gauge Indication
  • Water temperature indicator sender unit (10884).
  • Engine Coolant Temperature Gauge (10883).
  • Circuit.
  • Instrument cluster printed circuit.







Click below for testing, See Section C






Engine Coolant Temperature Sending Unit TESTING

  • Locate and remove the sender.
  • Place the sender in a container of water and heat the water to 176°F (80°C).
  • Using a high impedance Digital Volt Ohmmeter (DVOM), measure the resistance between the wire terminal and the case. The resistance should be between 49 and 58 ohms.
  • Measure the resistance at the same points as the water cools.
  • The resistance should gradually decrease to 24 ohms at 130°F (48°C), and then to 9.7 ohms at 70°F (22°C).
  • If the resistances do not meet specification, replace the sender.
  • If the resistances are as specified, check the circuit wiring for damage or a short to ground.





    Edited by Shawn on 11/17/2009 at 3:41 AM EST
    Customer: replied 7 years ago.
    Thank you for the info, I have a few follow up questions regarding your answer: you say to hoist the front of the car so the radiator is highest point on engine, would driving the front end up onto ramps be okay or do I have to go higher yet? Is it safe to have the engine running with the radiator cap open? How would you describe how to 'burp' the system, would it be by squeezing the big hose going from radiator to the engine? How do I know when to stop 'burping' the system? And to 'maintain the coolant level', would I add coolant directly to the radiator while the engine is running or into the overflow tank? On a side note, I notice that the coolant level in the overflow tank never really seems to change whether the engine is hot or cold, could that figure into this problem at all? I wonder if that coolant ever moves to and from that tank. Thanks.



    You are welcome, and yes ramps would be perfect.


    Yes you want to run the car with the cap open, otherwise the air will remain trapped, you can squeeze the radiator hose to hurry the process. And yes it is safe.


    You will need to maintain coolant level by adding coolant when needed, when the "burps" the coolant level may drop.

    Customer: replied 7 years ago.

    About how long does the burping process take and how will I know when it is enough? I've never 'burped' a coolant system before, does it make some sort of noise so I know that air is being let out of the system? How long can I leave the radiator cap off because won't coolant overflow out of that hole after awhile? If necessary, do I add coolant directly to the radiator during this process and do I try to keep the coolant level in the radiator topped off? I apologize for so many detailed questions, I just want to know exactly what I'm supposed to do. Thank you for your patience.



    When the car is cold the coolant in the radiator will stay dormant, until the coolant in the engine warms up and the thermostat opens allowing the coolant to begin to flow, once that happens and you will notice the coolant begin to flow and move and start circulating, it will force the air pockets that are trapped in the coolant, and engine to the surface. Once the coolant is flowing and starts overflowing, then cap it off.

    Edited by Shawn on 11/18/2009 at 1:45 AM EST
    Customer: replied 7 years ago.

    Last night, I burped the system using your instructions, and tried driving the car to see if it had any effect. It took about 15 minutes of highway driving and another 15 minutes of city driving to get it off of the cold mark on the gauge, but it did eventually go up to the halfway point between the C and H. Today I drove for about 30 minutes city and highway and it never rose above the C until I let it idle in my garage for another 10 minutes after that first 30 minutes. Would it be possible that cool air blasting through the engine compartment when driving at high speeds would have such an effect on the coolant temp reading that it would never get much above the C? Last night when the needle was at the halfway point between the C and H, I felt the radiator hose and it was very hot to the touch, and today when it was only at the C after 30 minutes, the hose was noticeably less hot, but still hot nonetheless, so it seems that the gauge is not giving me an entirely false reading but I would think the gauge would have read higher than it actually did. These results have happened before periodically BEFORE I burped the coolant system, typically on days that were not overly cold, and last night wasn't overly cold, so at this moment I'm not yet sure if last night was a fluke that it did work, or if the problem has truly been solved. Based on the info you gave me, along with the workshop manual pages, I will be happy to accept your answer, but I do have one more question. Are the instructions you listed on an earlier answer as to how to test the engine coolant temp sending unit (the test where you put the sensor in heated water and measure the ohms) for the sensor that sends a signal to the gauge or to the one that sends a signal to the PCM? Please let me know what you make of my response and I will be happy to accept you answer. Thank you for your help.



    Ok, no problem the following tests are for the sending unit which is for the gauge. To test the ECT for the PCM a NGS (New Generation Star) or equivalent scan tool is required


    Engine Coolant Temperature Sending Unit TESTING

  • Locate and remove the sender.
  • Place the sender in a container of water and heat the water to 176°F (80°C).
  • Using a high impedance Digital Volt Ohmmeter (DVOM), measure the resistance between the wire terminal and the case. The resistance should be between 49 and 58 ohms.
  • Measure the resistance at the same points as the water cools.
  • The resistance should gradually decrease to 24 ohms at 130°F (48°C), and then to 9.7 ohms at 70°F (22°C).
  • If the resistances do not meet specification, replace the sender.
  • If the resistances are as specified, check the circuit wiring for damage or a short to ground.

  • Edited by Shawn on 11/20/2009 at 3:56 AM EST



    Thank you for asking your question on JustAnswer.


    The other Experts and I are working on your answer. By the way, it would help us to know:

    -Did you need more help?

    Thank you again for trusting us with your problem. Please reply as soon as possible so that we can finish answering your question.

    Customer: replied 7 years ago.
    Is it possible for the radiator to radiate too much heat, thereby not allowing the coolant to get hot enough? Drove about 30 miles @ 60mph and never got beyond the cold mark until getting I drove 70mph for 5 miles, only then did it raise above the cold mark and it only went up a little. I still have to test the sensor and gauge.



    Not likely, either a bad sending unit, or gauge, you will need to perform the tests to determine.

    Customer: replied 7 years ago.

    The check engine light came on again from the coolant temp being too low. Got the gauge to go up to 1/2 way point after sitting for a few minutes after driving 1 hour, but dropped to C when resuming driving. Still seems to exhibit signs of bad thermostat if it causes ECT sensor to kick the check engine light on, is it possible for thermostat to be installed backwards or any way the coolant can get around the thermostat when it is not open (bad seal)? If that would be the case, wouldn't the coolant always be flowing even when the engine is still cold?



    It is unlikely the thermostat was installed incorrectly, but if it is in fact stuck open then you would see the same results you are getting now. Yes the coolant would flow.



    I am sending the instructions for the thermostat replacement in pdf format, (CLICK HERE FOR INSTRUCTIONS).

    Shawn and 3 other Ford Specialists are ready to help you
    Customer: replied 7 years ago.

    I changed out the thermostat and put the old one back in, and I replaced the O-ring around the thermostat with a new one and the problem seems to have gone away, so I'm not sure if it was the thermostat or the O-Ring, but I thought you may want to know. I am accepting your answer so that you get paid for responding to so many of my questions and for providing instructions for testing and installations. Thanks again for your help.

    Thank you very much!