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Mercury Monterey P0125: Insufficient Coolant Temperature for Closed Loop Fuel Control


P0125 Mercury Monterey

P0125 is a generic OBD-II code. If your Mercury Monterey has registered this, it means that the ECM/PCM is receiving a signal from the engine coolant temperature (ECT) sensor that the engine is not hot enough to enter closed-loop fuel control. The P0125 code is usually caused by a faulty thermostat, low coolant level, or faulty ECT sensor.

P0125 is usually caused by a bad thermostat, low coolant, or bad ECT sensor

P0125 Symptoms: Monterey Mercury

Mercury Monterey P0125 OBD-II

There are not always noticeable symptoms associated with P0125, especially if it is the ECT sensor that has gone bad. If it is a cooling system issue that is causing the code to appear, you may be experiencing no heat (stuck thermostat).

These are some of the symptoms you may notice with P0125:

  • Check the engine soon
  • Reduced fuel consumption
  • The heater is not working as well as it should (a very clear sign that the thermostat is bad)
  • The torque converter does not lock up or increase RPM on highway

P0125 Causes: Monterey Mercury

These are the most common causes why the P0125 code is registered:

Low coolant

Start your P0125 diagnosis by checking the coolant level. It will only take you a moment.

If your Monterey is very low on coolant, it may not have enough for the ECT sensor to register. If you have so little coolant that the code registers, the engine may be very hot, even though the ECT sensor reads low temperatures.

If the coolant level is very low, do not fill it when the engine is hot. Let it cool first. Introducing room-temperature coolant into an overheating engine can cause the head gasket to burst.

Bad thermostat or thermostat housing

If the coolant level is fine, the thermostat may be stuck. When this occurs, coolant is always circulating through your Monterey’s engine. The cooling system will keep the engine running cooler than it needs to be. This, in turn, will keep the coolant temperature too cold for closed-loop fuel control.

See also: Diagnosing Faulty Monterey Thermostat

You may notice that the heating is not working as well and that the temperature gauge is not as high as it should be.

Stuck cooling fan

If the cooling fan is running when it is not needed, it can cause the coolant to be so cold that the engine cannot reach the temperature at which it should operate.

Defective ECT sensor

If the ECT sensor has gone bad, coolant would continue to flow through the thermostat, and the coolant levels would be full and cycling. Once this is verified, it is very likely that the sensor has been damaged. A new ECT sensor is not that expensive.

Conclusion: P0125 Monterrey

Good luck diagnosing P0125 on your Mercury Monterey. If there is anything you would like to add, feel free to leave a comment below.


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