Insufficient Coolant Temperature For Closed Loop Fuel Control
Our emissions expert has put together the following information about the P0125 fault code. We have also included diagnostic procedures you can take to your repair shop if the mechanic is having difficulty analyzing the code.
OBD II Fault Code
- OBD II P0125
Fault Code Definition
- Insufficient Coolant Temperature For Close Loop Fuel Control
- Check Engine Light on
- Vehicle may not shift into the highest gear at freeway speed
- Decrease in fuel economy
- In unusual cases, there are no adverse conditions noticed by the driver
Common Problems That Trigger the P0125 Code
- Defective Engine Thermostat
- Defective Engine Coolant Temperature Sensor
- Defective Intake Air Temperature Sensor
- Defective Cooling System
- Low Engine Coolant
- Dirty Engine Coolant, causing incorrect Coolant Temperature Sensor readings
- Defective, always running Engine Cooling fan(s)
- Engine Cooling Fan
- Internal Engine problem
- Oxygen Sensor problem
- Engine Coolant Temperature Sensor problem
Polluting Gases Expelled
- HCs (Hydrocarbons): Unburned droplets of raw fuel that smell, affect breathing, and contribute to smog
- CO (Carbon Monoxide): Partially burned fuel that is an odorless and deadly poisonous gas
P0125 Diagnostic Theory for Shops and Technicians
Here's how to diagnose the P0125 code:
- It is critically important to record the freeze frame data to determine which operational mode set the code. Pay close attention to the MPH, TPS, LOAD, RPM, and of course, the Engine Coolant Temperature and Intake Air Temperature. These values will help determine if the vehicle was being driven at freeway or slower, in town driving speeds.
- Connect the scanner and select the most factory like data stream for the engine sensors. Start the vehicle (make sure the heater is off) and watch the Coolant Temperature values change.
- If the Coolant Temperature readings will not go above the 160–170º F mark within fifteen minutes, the thermostat is the most likely culprit.
- Be sure to verify that the engine coolant temperature readings agree with physical reality, so use a lazer pyrometer to measure the temperature of the engine, so you don't mistakenly blame the engine cooling system for a problem that is being caused by high resistance in the coolant temperature sensor, its connections or circuitry.
When doing the P0125 diagnosis, be sure to verify that either the electric or mechanical cooling fan is not stuck on because this will cause the engine to run at a very reduced operating temperature. Also, be sure to verify that the Intake Air Temperature Sensor readings are within reason, meaning that they are not too hot or too cold in relation to the outside air temperature and under-hood air temperature. A good rule of thumb is that the Intake Air Temperature reading is usually about 100º F below the Coolant Temperature readings after warm up. When cold starting the vehicle, the readings should begin at nearly identical values.