P0125: Insufficient coolant temperature for closed loop fuel control
The engine takes too long to get warm enough for the engine computer to start steering the fuel mixture by the oxygen sensor (closed loop). Until then it runs a preset, richer mixture.
Updated
What P0125 means
When the engine is cold, the engine computer runs a preset, richer mixture. Once the coolant is warm enough, it switches to closed loop: it corrects the mixture using the oxygen sensor.
P0125 means the coolant took too long to reach that temperature. The usual reason is a thermostat stuck open, which lets the coolant through the radiator before the engine is warm. A sensor that reads too cold can set it too.
On some cars, very cold weather with only short trips can set it as well. If it comes back in normal conditions, have it checked.
P0126: not warm enough for stable operation
P0126 is a similar check: the coolant stays below the temperature the engine computer expects for stable running. The causes and advice are the same, with the thermostat the most common cause.
P0126: Insufficient Coolant Temperature for Stable Operation
The coolant temperature stays below what the engine computer expects for stable running, after the engine should have warmed up.
Common causes: thermostat stuck open, faulty coolant temperature sensor, low coolant level.
Can I keep driving?
Check soon
Probably OK to drive short-term, but have it checked soon. Fuel use or emissions may be worse. Check the coolant level with the engine cold first: if it’s low, don’t drive until it’s topped up and the leak is found, because a sensor without coolant around it can read cold while the engine overheats.
General guidance based on the code alone, not on the car’s actual condition. If in doubt, stop and ask a mechanic.
Until the engine warms up it uses more fuel, and the heater may stay cool. If the coolant level is low, find out why: a leak can lead to overheating.
Symptoms
- The temperature gauge stays low, or takes a long time to rise.
- The heater is slow to blow warm air.
- Higher fuel use.
- Often P0128 stored as well.
Common causes
- Thermostat stuck open. The most common cause: coolant flows through the radiator from the start, so the engine warms up slowly, especially in cold weather.
- Faulty coolant temperature sensor. A sensor that reads too cold makes a warm engine look cold.
- Low coolant level. Check it with the engine cold. A sensor without coolant around it reads wrong, and a leak needs fixing before the engine overheats.
Typical fixes
- With the engine cold, check the coolant level.
- Compare the coolant temperature in live data with the outside temperature on a cold engine, to check the sensor.
- Watch the coolant temperature in live data as the engine warms up: it should rise steadily and then hold steady once warm. If it creeps up slowly, or drops when driving at speed, suspect the thermostat.
- Replace the thermostat or the sensor, whichever is at fault, then clear the code.
Questions
Can I drive with P0125?
Probably for a short time, but have it checked soon: fuel use or emissions may be worse. Check the coolant level with the engine cold first. If it’s low, don’t drive until it’s topped up and the leak is found: a sensor without coolant around it can read cold while the engine overheats.
What is the difference between P0125 and P0128?
Both usually point to the thermostat. P0125 means the engine took too long to get warm enough for closed-loop fuel control; P0128 means it stays below the temperature the thermostat should hold. They often come together.