P0125 and P0126: coolant too cold for fuel control or stable running
The engine computer saw the coolant stay too cold for too long: not warm enough to start using the oxygen sensors for fuel control (P0125), or not warm enough for stable running (P0126).
SeverityModerateUsually not urgent today, but it can get worse if left.
Can you keep driving?Drive gently, check it soonWhat does P0125 mean?
A cold engine runs on a richer mixture without listening to its oxygen sensors. Once the coolant reaches a set temperature, the computer switches to closed loop and trims the mixture by the sensors. If that temperature arrives too late, the engine spends longer running rich, using more fuel and producing more emissions. The usual cause is a thermostat stuck open, though a faulty coolant temperature sensor can give the same result.
What causes P0125?
- A thermostat stuck open or opening too early
- A coolant temperature sensor that reads low
- Low coolant around the sensor
- Very cold weather combined with short trips
What to check first for P0125
- Watch how long the temperature gauge takes to reach its normal position
- Compare the coolant temperature on a scan tool with the real engine temperature
- Check the coolant level in the reservoir before the first start of the day
- Feel whether the cabin heater gets properly hot
Can you drive with P0125?
You can drive to a workshop. Fuel use goes up and the heater may stay lukewarm, so have the thermostat checked soon, and watch the gauge in case the cause is low coolant.
Since May 2018 an engine warning light that stays on is a major defect at the MOT, so the car fails until the fault is fixed.
Codes on this page
- P0125Insufficient Coolant Temperature for Closed Loop Fuel Control
- P0126Insufficient Coolant Temperature for Stable Operation
Related codes
Checked on · Rafael Soares
These are the generic meanings every manufacturer shares. Each brand adds its own detail, so your car's handbook, its service information and a qualified mechanic have the last word. OdoKeep has no connection with any manufacturer, and the app does not read codes from the car.
