P0125 – Insufficient Coolant Temperature for Closed Loop Fuel Control

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The trouble code P0125 is a generic OBD-II code that will set when the time it takes for the vehicle’s engine to reach operating temperature is longer than a specified value. While you may not notice any drivability symptoms with the P0125 code, it can cause your car not to enter closed loop operation soon enough or at all, causing your emissions levels to rise and fail a state emissions test.

P0125 Definition

P0125 (Generic): Insufficient Coolant Temperature for Closed-Loop Operation

What Does P0125 Mean?

The P0125 code indicates that the ECM is not operating properly
The P0125 code means the engine isn’t reaching enough temperature for the ECM to operate properly

For your vehicle to run as clean and efficiently as possible, the engine needs to reach the proper operating temperature as soon as possible. The engine coolant temperature sensor monitors the coolant’s temperature in the engine’s cooling system and is a primary input to the ECM. Once the ECT senses that the coolant is at a specified temperature, it allows the ECM to operate in a “closed loop”. The ECM then uses all the inputs from various sensors to modify different engine operating parameters to maximize fuel efficiency, engine power, and emission levels. A P0125 trouble code indicates that the engine is not reaching this temperature.

What Are The Symptoms Of The P0125 Code?

You may not notice any symptoms with a P0125 trouble code other than an illuminated check engine light and a temperature gauge that is reading lower than usual. You may notice a slight drop in fuel mileage or poor heater performance. If left unresolved, running at too low of an engine temperature for an extended time can result in excessive carbon build-up.

What Are The Causes Of P0125?

  • Faulty Thermostat
  • Low Coolant Level
  • Defective Engine Coolant Temperature Sensor
  • Wiring Problem

How Serious Is The P0125 Code?

P0125 is not a very serious trouble code, although it should be repaired as soon as possible. Continuing to operate below operating temperature can cause lower fuel mileage, excess carbon build-up, and higher than average exhaust emissions.

How To Diagnose And Fix The P0125 Trouble Code

To diagnose code P0125:

  1. Start with connecting your OBD-II scan tool and recording all trouble codes and freeze frame data. If there are codes relating to the engine coolant temperature sensor, diagnose and repair them first.
  2. Once you have recorded and cleared trouble codes, check the coolant level. If it is low, check for and repair any leaks. In case it is full, start the engine and allow it to warm up.
  3. Using an infrared thermometer, compare the engine’s actual temperature with the data PID for ECT on your scan tool. If they match, it is an indication that the ECT sensor is functioning correctly and the associated wiring is intact. If not, visually inspect the wiring harness and connectors for broken, chafed, or corroded wires. 
  4. If the ECT sensor and wiring check out OK, again, using your infrared thermometer, check the different hoses’ temperature in the cooling and heating systems. 
  5. If they are generally the same temperature and the engine has been running for at least 15 minutes, the coolant thermostat is the most likely cause.
How to fix the P0125 code

Common Mistakes To Avoid While Diagnosing The P0125

Some common mistakes to avoid while diagnosing a P0125 code are jumping straight to the thermostat and not following the full diagnostic procedure. And after a coolant leak is repaired or a thermostat is replaced, not properly bleeding the cooling system is another common mistake.

Tips To Avoid P0125 In The Future

To help avoid P0125 in the future, be sure to follow the manufacturer’s maintenance schedule, particularly in coolant flush intervals.

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