P0125
Insufficient Coolant Temperature for Closed Loop Fuel Control
Low· Fix when convenientDIY friendlyPowertrainGeneric, all makes
What it means
The engine is taking too long to warm up to the temperature where the ECU starts using the oxygen sensors to control the fuel mixture ("closed loop"). This is most often a thermostat stuck open.
Is it safe to drive?
Safe to drive. Expect worse fuel economy and a weak heater.
Symptoms
- Check engine light on
- Temperature gauge stays low
- Heater blows lukewarm air
- Poor fuel economy
Probable causes
Most likely first
- 1Thermostat stuck open
- 2Faulty coolant temperature sensor
- 3Low coolant level
- 4Cooling fan running all the time
How to diagnose it
Work through these in order before buying parts
- 1Record the code and its freeze frame data (speed, load, temperature when it set), then clear it and see if it comes back.
- 2Check the coolant level when cold.
- 3Watch coolant temperature on live data from a cold start; it should reach about 80 °C (176 °F) within 10–15 minutes of driving.
- 4Feel the upper radiator hose: if it warms up at the same time as the engine, the thermostat is open too early.
- 5Check for technical service bulletins (TSBs) or software updates for your make and model; some codes have known factory fixes.
Common fixes
- Replace the thermostat
- Replace the coolant temperature sensor
- Top up and bleed the coolant
This information is general guidance for all makes. Exact causes, specifications and procedures vary by vehicle, so confirm with your service manual or a qualified mechanic before replacing parts.