Toyota Tundra P0125: Insufficient Coolant Temp for Closed Loop Fuel Control

P0125 Toyota Tundra

P0125 is a generic OBD-II code.  If your Toyota Tundra has registered it, means that the ECM/PCM is getting a signal from the engine coolant temperature sensor (ECT) that the engine is not warm enough to enter closed loop fuel control.  P0125 is usually caused by a bad thermostat, low coolant, or a bad ECT sensor.


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


P0125 Symptoms:  Toyota Tundra

Toyota Tundra P0125 OBD-II


There are not always any noticeable symptoms associated with P0125, particularly if it’s the ECT sensor that has gone bad.  If it’s a cooling system issue that is causing the code to appear, you may be experiencing a lack of heating (thermostat stuck open).

Here are some of the symptoms that you may notice with P0125:

  • Service engine soon
  • Decreased fuel mileage
  • Heater not working as well as it should (very strong indication of a bad thermostat)
  • Torque converter not locking or higher highway RPM


P0125 Causes:  Toyota Tundra

Here are the most common causes for the P0125 code to register:


Low Coolant

Start your P0125 diagnosis by checking the coolant level.  It only takes a moment.

If your Tundra is really low on coolant, it may not have enough for the ECT sensor to register.  If it is so low on coolant that the code is registering, the engine can be really hot, even though the ECT sensor is reading low temps.  

If the coolant level reads bone dry, don’t fill it up when the engine is hot.  Let it cool down first.  Introducing room temp coolant to an engine that is overheating can cause a head gasket to blow.


Bad Thermostat or Thermostat Housing

If the coolant level is fine, the thermostat may be stuck open.  When this happens the coolant is always cycling through your Tundra’s engine.  The cooling system will keep the engine running cooler than it needs to run.  This in turn will keep the coolant temperature too cool for closed loop fuel control.  

See Also:  Tundra Bad Thermostat Diagnosis

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


Cooling Fan Stuck

If the cooling fan is running when it is not needed, it can cause the coolant to be so much cooler the engine can’t get to the temperature that it needs to run at.


Bad ECT Sensor

If the ECT sensor has gone bad, coolant would still be flowing through the thermostat, and the coolant levels would be full and cycling.  Once you verify that, it’s highly likely that the sensor has gone bad.  A new ECT sensor isn’t that expensive.  


Conclusion:  P0125 Tundra

Good luck diagnosing P0125 in your Toyota Tundra.  If there is anything that you would like to add, please feel free to leave a comment below.