P0125 – Insufficient coolant temperature for closed loop fuel control

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By Benjamin Jerew (Contact Me)
Last Updated 2016-07-12
ASE Master Tech
CodeFault LocationProbable Cause
P0125 Insufficient coolant temperature for closed loop fuel control
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Wiring, cooling system, coolant thermostat, ECT sensor

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Table of Contents

  1. What Does Code P0125 Mean?
  2. What are the common causes of code P0125 ?
  3. What are the symptoms of code P0125 ?
  4. How do you troubleshoot code P0125 ?
  5. Codes Related to P0125
  6. Get Help with P0125

What Does Code P0125 Mean?

To extract the best performance from a given drop of fuel, the modern engine monitors and modulates a number of things, including oxygen content, spark timing, valve timing, injector pulse width, and many others. Among all of these, temperature is very important, both of the fuel itself and the intake and cylinders. Heat, of course, is generated by the combustion of the fuel, but it must be maintained within a certain range.

ECT Circuit
ECT Circuit

Within the correct temperature range – depending on the vehicle, “operating temperature” is usually somewhere between 160 °F and 180 °F – fuel vaporizes efficiently, which enables complete combustion. Higher temperatures improve fuel vaporization, but only to a certain point. If temperature is too high, this can lead to engine ping, detonation, even damage. On the other hand, if temperature is too low, fuel doesn’t vaporize or combust as efficiently, which means that the engine control module (ECM) has to inject more fuel than normal.

Until the ECM determines that the engine has reached operating temperature, it runs the engine in “open loop,” that is, following pre-programmed parameters (fuel injection, spark advance, valve timing, etc.) for the given engine speed, load, and driver demand. In open loop mode, the ECM does not use feedback from the AFR sensors to fine-tune performance and emissions. Once the engine reaches operating temperature, the ECM goes into “closed loop” mode, using the AFR sensors to maintain performance and fuel economy.

Generally, the ECM expects that the engine will take a certain amount of time to reach proper operating temperature, but this can vary, depending on climate, coolant level, and thermostat function. For example, if ambient temperature is 0 °F or below, the ECM may look for engine temperature to rise at least 70 °F within 15 minutes. At higher ambient temperatures, the engine should reach operating temperature, 160 °F to 180 °F, within 15 minutes. It seems counterintuitive, but the ECM doesn’t necessarily engine warming based entirely on ECT sensor readings, but also on the reaction of the air fuel ratio (AFR) sensors, as these are also active within a certain temperature range.

If the ECM detects, within a certain amount of time, that the engine has not reached proper operating temperature and can’t go into closed loop mode, it interprets this as a fault. The ECM will set a diagnostic trouble code (DTC) in memory and may or may not command the malfunction indicator lamp (MIL) to illuminate. DTC P0125 is defined as “Insufficient Coolant Temperature for Closed Loop Fuel Control.”

What are the common causes of code P0125 ?

Depending on year, make, and model, DTC P0125 may have number of causes. Here are some of the most common.

  • AFR Sensor – If the AFR sensor is stuck lean, has a faulty heater, or is exposed to an exhaust leak, this can trick the ECM into thinking that the engine has not reached operating temperature.
  • Typical AFR Signal
    Typical AFR Signal

    Toyota TSB – Toyota has released at least a couple of TSBs referencing DTC P0125. Barring other cooling system problems, the fix requires AFR sensor replacement or ECM reprogramming. Always check your dealership service center for relevant TSBs before starting diagnosis.

  • Cold Weather – In some cases of extreme cold, even with a properly filled cooling system and functional thermostat, the engine simply may not be able to build up enough heat to reach operating temperature. Consider adding a grille cover and additional underhood insulation and coverings to help maintain engine heat. A block heater may also be helpful in certain situations.
  • Engine Coolant Level – If there is a leak, faulty coolant return, or the system wasn’t properly bled after a coolant change, the engine may not heat properly.
  • Broken Thermostat – If the thermostat is broken, it may allow coolant to flow all the time, that is, always taking heat away from the engine.
  • ECT Sensor – If there is corrosion in the ECT sensor circuit, or the ECT sensor is faulty, this may give the ECM false engine coolant temperature readings.

What are the symptoms of code P0125 ?

Whether or not the MIL comes on, at the very least you will notice decreased fuel economy. You may also experience decreased engine performance. There may be related DTCs in ECM memory, as well.

How do you troubleshoot code P0125 ?

Again, this DTC may be a little tricky to diagnose, as most automakers use both ECT and AFR sensor readings to make this determination. If there are concurrent AFR or ECT DTCs, address these first, and then move forward with diagnosis of DTC P0125. Also, check your dealership service center for relevant TSBs (technical service bulletins) that may address your issue.

  • Broken Thermostat Always Allows Coolant Flow
    Broken Thermostat Always Allows Coolant Flow

    Coolant Check – Make sure that coolant level is correct, both in the radiator (when cold) and in the coolant overflow reservoir. Be sure there are no leaks and that the pressure cap is working properly. If there is insufficient coolant in the system, the engine may not warm properly. If the ECT is not in contact with the coolant, it will not read the temperature. Similarly, if there is a problem with the ECT sensor or circuit, the ECM may not receive a true indication of the engine coolant temperature.

  • Thermostat Check – Using a non-contact thermometer, your hands, or a scan tool with live data streaming, check to see if the engine is warming up properly. Normally, the radiator top hose should stay relatively cool until the thermostat opens, at which point it will heat up rapidly. If the thermostat is open, coolant will flow constantly, never allowing the engine to reach operating temperature.
    • Using the thermometer or scan tool, verify engine temperature after the engine has been running 10-15 minutes. If the temperature doesn’t reach operating temperature, check the ECT sensor, wiring, coolant level, and thermostat operation.
  • AFR Signal Analysis
    AFR Signal Analysis

    AFR Sensor Check – You can try to use a DMM (digital multimeter) to watch AFR voltage, but a scan tool with live data streaming is a much better option. You need to see if the AFR voltage fluctuates, more or less in a sine-wave pattern on the scan tool screen or high and low voltage on the DMM (depending on the DMM, you may not even be able to see this). Depending on the engine, the AFR signal should fluctuate between 1 V and 4 V or 0.1 V and 0.9 V.

    • If the AFR signal fluctuates, this means that the AFR and ECM are controlling fuel trim at this time and that there are no problems in the sensor.
    • If the AFR signal is stuck at low voltage, a “lean” fuel mixture, never rising higher than 2.5 V or 0.5 V to indicate a “rich” fuel mixture, suspect a bad AFR sensor. Further diagnosis will be required to determine if the AFR sensor or exhaust is faulty. Exhaust leaks, for example, may allow atmospheric oxygen into the exhaust stream, which can skew AFR sensor readings.
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  • P0126 Insufficient Coolant Temp. for Stable Operation
  • P0128 Coolant Thermostat (Coolant Temp. Below Thermostat Regulating Temp.)
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  • P0153 O2 Sensor Circuit Slow Response (Bank 2 Sensor 1)
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  • P2197 O2 Sensor Signal Biased/Stuck Lean Bank 2 Sensor 1

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