What is closed loop fuel control?

It is the mode in which the engine adjusts its own fuelling from live feedback. The oxygen sensor in the exhaust reports whether the last mixture was rich or lean, and the control unit corrects continuously to hold the chemically correct ratio. Before the engine is warm it cannot do this, because a cold engine needs extra fuel and the oxygen sensors are not yet reliable, so it runs open loop on a fixed enriched map instead. This code means the engine never got warm enough to make that switch, so it has been running on the enriched map for the whole journey.

What is the difference between P0125 and P0128?

They share their causes but describe different thresholds and different consequences. P0128 means the coolant did not reach the temperature the thermostat should be regulating at, which is a statement about the cooling system. P0125 means the coolant did not reach the temperature at which the control unit will begin trusting the oxygen sensors, which is a statement about fuelling. They very often appear together, and a stuck-open thermostat or a coolant temperature sensor reading low explains both, but P0125 is the one that tells you the mixture is not being controlled at all.

Will P0125 fail an MOT?

Yes, and it is one of the codes most likely to fail on the emissions readings rather than only on the lamp. The tester requires the calculated lambda at fast idle to sit between 0.97 and 1.03, and that band is essentially a measurement of closed-loop mixture control working — which is exactly what this code says is not happening. On top of that, the test must be carried out with the engine at normal operating temperature, and a car that cannot reach it may not be testable at all, which is a major failure in itself. The illuminated lamp is a major defect separately.

Why does my fuel consumption get so bad with a P0125?

Because the engine is stuck on its warm-up settings. The cold-start map deliberately delivers considerably more fuel than a warm engine needs, and it is supposed to be a temporary arrangement lasting a minute or two. If the engine never reaches the temperature that ends it, that enrichment applies to every mile of every journey. Short trips are worst affected, since the engine spends its entire time in the mode it should have left almost immediately.

Powertrain code · standard

P0125Engine too cold for the fuelling system to work properly

Standard definition: Insufficient Coolant Temperature for Closed Loop Fuel Control

Last updated 27 July 2026

Quick answer

The engine has not reached the temperature at which the control unit is willing to trust its oxygen sensors, so it is still running on a fixed warm-up map with extra fuel. It shares its causes with the thermostat code but describes a different consequence: not just a cold engine, but an engine whose mixture is not being controlled at all.

How the car works this out

A petrol engine starts on a pre-programmed enriched map because a cold engine needs extra fuel and because the oxygen sensors cannot be relied upon until everything is warm. Once the coolant passes a threshold temperature, the control unit switches to closed-loop operation and begins trimming fuelling from the oxygen sensor signal. This code is set when the coolant reading does not reach that threshold within the expected time, so the switch to closed loop never happens. The measurement is the same one behind the thermostat code, but the threshold and the consequence are different — this is about the point at which mixture control can begin.

What you might notice

Fuel consumption clearly worse than usual, often dramatically so on short journeys
A rich exhaust smell and, on a petrol, sooty deposits at the tailpipe
The heater never getting properly hot
The temperature gauge sitting low or taking an age to rise
A slight stumble or roughness that never quite clears as the journey goes on

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1A thermostat stuck open, so the coolant never reaches the temperature the control unit is waiting forCommon
How to check: From a cold start, the top radiator hose should stay cool for several minutes and then heat suddenly. Gradual warming from the outset means coolant is circulating when it should be held back.
2A coolant temperature sensor reading low or drifting, so the control unit believes the engine is colder than it isCommon
How to check: This cause matters more here than for the thermostat code, because the decision to enter closed loop rests directly on this reading. Compare the reported temperature with the actual temperature of the thermostat housing once the engine is fully warm; a sensor reading well below the metal is the fault, and it is an inexpensive part.
3A low coolant level leaving the sensor measuring air rather than coolantPossible
How to check: Check when cold, and look for the leak if it needs topping up.
4A damaged connector or corroded wiring at the coolant temperature sensor, adding resistance that the control unit reads as a lower temperaturePossible
5Very cold weather combined with repeated short journeys on a marginal thermostat, which can set the code seasonally on a car that is otherwise finePossible
6The wrong thermostat rating fitted during an earlier repairLess common

What to do about it

1Have the coolant temperature sensor's reading verified against the actual metal temperature. It is the cheapest component involved and the one the control unit's decision hangs on directly.
2Check the coolant level and the thermostat's behaviour from cold. Between the sensor and the thermostat you have almost all of the likely causes.
3Do not dismiss it as a winter quirk. Running open loop means the engine is being fed a fixed enriched mixture for the whole journey, which wastes a great deal of fuel and puts unburnt fuel through the catalytic converter.
4Change the oil after a prolonged period of running rich, since extended enrichment dilutes the sump oil with petrol.
5Sort it before an MOT. Of all the emissions codes, this is one of the most likely to fail the gas test in its own right.

Can you keep driving?

Yes

The car is safe and will drive normally, if thirstily. The concerns are cumulative rather than immediate: a permanently enriched mixture wastes fuel on every journey, washes lubrication off the cylinder bores, dilutes the engine oil and works the catalytic converter far harder than it should be worked. None of that is dangerous this week, but the converter is the expensive part in that sentence.

Will it fail an MOT?

This code has one of the strongest direct claims on the emissions test of any fault code, and it is worth being precise about why. For a catalyst-equipped petrol car the tester calculates lambda from the tailpipe sample at fast idle and it must fall between 0.97 and 1.03. Lambda sitting inside that window is essentially the signature of closed-loop mixture control working — and this code is the control unit telling you that closed loop was never entered. An engine fuelling from a fixed enriched map has nothing holding it in that band, so the car has a real prospect of failing on gas even before the lamp is considered. The lamp is a major defect in its own right on cars within the rule, and there is a third route: the emissions test must be carried out with the engine at normal operating temperature, and if it cannot get there the test cannot be properly completed, which is a major failure as well.

The light this turns on

Coolant temperature warning lightThe engine is running hotter than it is designed to. This is one of the few dashboard warnings that can destroy an engine in the time it takes to finish a journey, so it means stopping rather than pressing on.

Common questions

What is closed loop fuel control?: It is the mode in which the engine adjusts its own fuelling from live feedback. The oxygen sensor in the exhaust reports whether the last mixture was rich or lean, and the control unit corrects continuously to hold the chemically correct ratio. Before the engine is warm it cannot do this, because a cold engine needs extra fuel and the oxygen sensors are not yet reliable, so it runs open loop on a fixed enriched map instead. This code means the engine never got warm enough to make that switch, so it has been running on the enriched map for the whole journey.
What is the difference between P0125 and P0128?: They share their causes but describe different thresholds and different consequences. P0128 means the coolant did not reach the temperature the thermostat should be regulating at, which is a statement about the cooling system. P0125 means the coolant did not reach the temperature at which the control unit will begin trusting the oxygen sensors, which is a statement about fuelling. They very often appear together, and a stuck-open thermostat or a coolant temperature sensor reading low explains both, but P0125 is the one that tells you the mixture is not being controlled at all.
Will P0125 fail an MOT?: Yes, and it is one of the codes most likely to fail on the emissions readings rather than only on the lamp. The tester requires the calculated lambda at fast idle to sit between 0.97 and 1.03, and that band is essentially a measurement of closed-loop mixture control working — which is exactly what this code says is not happening. On top of that, the test must be carried out with the engine at normal operating temperature, and a car that cannot reach it may not be testable at all, which is a major failure in itself. The illuminated lamp is a major defect separately.
Why does my fuel consumption get so bad with a P0125?: Because the engine is stuck on its warm-up settings. The cold-start map deliberately delivers considerably more fuel than a warm engine needs, and it is supposed to be a temporary arrangement lasting a minute or two. If the engine never reaches the temperature that ends it, that enrichment applies to every mile of every journey. Short trips are worst affected, since the engine spends its entire time in the mode it should have left almost immediately.

Check any vehicle's MOT history

Free, instant, and straight from DVSA records. No sign-up.