P0125: Insufficient Coolant Temperature for Closed Loop Fuel Control
The engine is taking too long to warm up enough for the computer to start using oxygen sensor feedback for fuel control.
What P0125 means
Your engine computer runs in open loop, using preset fuel tables, until the engine is warm enough to trust the oxygen sensors. RepairPal explains that it expects the coolant to reach about 160 to 170 degrees F within roughly 15 minutes of a stone cold start, meaning the car sat at least 8 hours. In very cold weather it looks for a rise of at least 70 degrees instead. If the engine does not get there, the computer cannot switch to closed loop fuel control, so it sets P0125 and keeps running on its rough backup program, which raises emissions. A thermostat stuck open is the usual cause, followed by low coolant, a bad temperature sensor and a fan that runs when it should not. RepairPal also notes some vehicles will not shift into the highest gear while this is happening. Shops log the warm up curve with a scan tool and check the thermostat. Fixing it improves heat, mileage and emissions.
Common symptoms
- Check engine light on
- Heater blows lukewarm air
- Temperature gauge stays low
- Lower fuel economy
- Transmission may not shift into top gear on some vehicles
Most common causes
- Thermostat stuck open or opening too early
- Low coolant level
- Faulty coolant temperature sensor
- Wiring problem in the ECT circuit
- Cooling fan stuck on
What to check yourself first
Check the coolant level when the engine is cold. After a 15 minute drive, the upper radiator hose should be hot and pressurized; if it is only warm, the thermostat may be stuck open. Check whether the radiator fan runs on a cold engine, which it should not on most cars.
Typical fixes and what they cost
| Fix | Typical cost | Note |
|---|---|---|
| Replace thermostat | $616-$716 | The most common fix, especially in cold weather. |
| Replace engine coolant temperature sensor | $499-$585 | If the engine is warm but the sensor reports it as cold. |
| Replace radiator fan relay | $98-$116 | If a stuck relay keeps the radiator fan running all the time. |
| Coolant drain and refill | $440-$512 | If the level is low or there is air in the system after a repair. |
Typical US price ranges for parts and labor, not quotes. Your shop should confirm the cause with a diagnostic before replacing parts.
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Related codes
- P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
- P0116 Engine Coolant Temperature Sensor 1 Circuit Range/Performance
- P0115 Engine Coolant Temperature Sensor 1 Circuit
- P0118 Engine Coolant Temperature Sensor 1 Circuit High
- P0480 Fan 1 Control Circuit