P0125 P0xxx · Powertrain (P) Low severity
Insufficient Coolant Temperature for Closed Loop Fuel Control
The engine has been running long enough that it should be in closed-loop fuel control, but the ECU never sees the coolant reach the expected temperature. Almost always a thermostat that is stuck open.
Severity guidance: Low — usually no drivability impact, fix at the next service
Symptoms you will notice
- Heater blows lukewarm
- Temperature gauge below normal
- Higher fuel consumption
- Emissions test failure
Common causes
Ordered from most to least likely — start at the top.
- 1 Thermostat stuck open
- 2 Failed coolant temperature sensor reading low
- 3 Very cold ambient use with a short journey cycle
How to diagnose it
Cheapest and least invasive first — most of these codes are not a parts problem.
- ✓ Watch ECT on a scan tool during a 10-minute drive; it should stabilise around 85–95 °C.
- ✓ Feel the upper and lower radiator hoses — both getting warm early means the thermostat is open.
- ✓ Compare ECT with a infrared thermometer on the housing.
Repair options
- Replace the thermostat
- Replace the ECT sensor if it reads implausibly low
- Bleed the cooling system
P0125 — frequently asked questions
Is this urgent? +
Not mechanically urgent, but running rich long-term damages the catalytic converter and washes oil films off cylinder walls.
Could it just be a cold day? +
In sub-zero conditions with short trips, yes — confirm with a longer drive before replacing parts.
Confirm the diagnosis before you buy parts
Send us the code, freeze-frame data and the vehicle — our engineers will tell you what to test first.