Does a stuck open thermostat fail an MOT?

It can, though not as a defect in its own right — there is no MOT check on the thermostat or the cooling system's performance. The problem is procedural: the emissions test must be carried out with the engine at normal operating temperature, and a car that will not get there either takes a very long time to test or cannot be tested properly, and an emissions test that cannot be completed is recorded as a major failure. A cold-running engine also produces higher carbon monoxide and hydrocarbon readings. If the fault has also lit the engine management lamp, that is a straightforward major defect on cars within the lamp rule.

Why is my heater not getting hot with a P0128 code?

Because the heater matrix is warmed by the same coolant the engine is failing to heat. With the thermostat stuck open, coolant circulates through the radiator continuously and never builds the temperature the heater needs, so the air coming through the vents is warm at best. It is usually worst at speed, when airflow through the radiator is greatest, which is why people notice it on the motorway. A weak heater in winter is more than an inconvenience, since a demister that cannot clear the screen quickly is a visibility problem.

Can I drive with a P0128 code?

Yes. This is the opposite of an overheating fault and nothing is at risk of sudden damage. What you are paying is a continuous cost: more fuel on every journey, additional engine wear from running below the designed temperature, and a heater that cannot do its job. Get it fixed at your convenience rather than treating it as an emergency, but do not leave it indefinitely, particularly with an MOT due.

Is P0128 expensive to fix?

The part is generally inexpensive — a thermostat is a small component and often comes with its housing and gasket. What varies enormously is access. On some engines it is at the end of the top hose and the job takes under an hour; on others it sits behind ancillaries or under the inlet manifold and the labour dominates. Ask for a quote that separates parts from labour before agreeing, and have the coolant level and the temperature sensor checked at the same time since both can produce this code without the thermostat being at fault.

Powertrain code · standard

P0128Engine not reaching normal running temperature

Standard definition: Coolant Thermostat Below Regulating Temperature

Last updated 27 July 2026

Quick answer

The engine is taking too long to warm up, or never gets properly warm at all, and a thermostat stuck open is nearly always the reason. It is a cheap fault with disproportionate consequences: worse economy all winter, a heater that never quite works, and an MOT emissions test that may be impossible to carry out correctly.

How the car works this out

The control unit does not measure the thermostat. It models how quickly the engine ought to warm up given the ambient temperature, how long it has been running, how much air is passing through the radiator and how hard it has been working, then compares that model against the actual reading from the coolant temperature sensor. If the coolant has not reached the temperature the thermostat should be regulating at within the expected time, this code is stored. It is a comparison between predicted and actual warm-up, which is why it appears far more readily in cold weather and on short journeys.

What you might notice

The temperature gauge taking a very long time to rise, or never reaching its normal position
A heater that blows warm rather than hot, at its worst on cold mornings and on the motorway
Fuel consumption noticeably worse, particularly in winter
The engine management lamp appearing only during cold spells and going out when the weather turns
On some cars, a cooling fan running more than seems reasonable

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, or opening at too low a temperature, so coolant circulates through the radiator before the engine has warmedCommon
How to check: Feel the top radiator hose from a cold start. It should stay cool for several minutes and then get hot quite suddenly as the thermostat opens. A hose that warms up gradually from the very beginning means coolant is circulating when it should not be.
2A coolant temperature sensor reading lower than the actual temperaturePossible
How to check: Compare the temperature the control unit reports at full operating temperature against the actual temperature of the thermostat housing measured with an infrared thermometer. A sensor reading well below the metal is the fault.
3A low coolant level, so the sensor is measuring air pockets rather than coolantPossible
How to check: Check the level when cold. It is free, and a system that keeps emptying has a leak that needs finding anyway.
4The wrong thermostat fitted at an earlier repair, opening at a lower temperature than the engine was designed forPossible
5A cooling fan running continuously because of a separate fault, pulling heat out faster than the engine can make itLess common
6Air trapped in the cooling system after a coolant change that was not bled properlyLess common

What to do about it

1Check the coolant level first. It takes a minute, it is free, and a low level explains both the code and a great many other things.
2Have the thermostat assessed. It is a modestly priced part on most engines, though on some the housing is buried and the labour is the bulk of the job — worth asking for a quote that separates the two.
3Do not ignore it because the car drives fine. An engine running below its designed temperature wears faster, uses more fuel, and produces more emissions every single journey.
4Sort it before booking an MOT if you can. An engine that will not reach operating temperature can make the emissions test difficult or impossible to complete properly.
5If the thermostat is replaced, make sure the correct temperature rating is used and the system is bled properly afterwards.

Can you keep driving?

Yes

This is the opposite of an overheating problem and it carries none of that urgency — an engine running cool is not about to destroy itself. The costs are steady rather than sudden: more fuel used on every journey, more wear from an engine spending too long at warm-up viscosity and enrichment, and a heater and demister that never quite do their job, which in winter has its own safety implications.

Will it fail an MOT?

The MOT emissions test requires the engine to be at normal operating temperature before any reading is taken, and that requirement is what makes this modest fault a genuine test problem. A car that cannot reach temperature leaves the tester either running it for a long time to try to get there, or unable to carry out the test properly at all — and an emissions test that cannot be completed is a major failure in exactly the same way as one that is completed and failed. On top of that, an engine held below temperature runs an enriched warm-up mixture for far longer than it should, which pushes carbon monoxide and hydrocarbons up at the probe. Note that this code does not usually light the red temperature warning lamp: that lamp is for overheating, and a cold-running engine will show only the engine management lamp, which is itself a major defect on cars within the lamp rule.

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

Does a stuck open thermostat fail an MOT?: It can, though not as a defect in its own right — there is no MOT check on the thermostat or the cooling system's performance. The problem is procedural: the emissions test must be carried out with the engine at normal operating temperature, and a car that will not get there either takes a very long time to test or cannot be tested properly, and an emissions test that cannot be completed is recorded as a major failure. A cold-running engine also produces higher carbon monoxide and hydrocarbon readings. If the fault has also lit the engine management lamp, that is a straightforward major defect on cars within the lamp rule.
Why is my heater not getting hot with a P0128 code?: Because the heater matrix is warmed by the same coolant the engine is failing to heat. With the thermostat stuck open, coolant circulates through the radiator continuously and never builds the temperature the heater needs, so the air coming through the vents is warm at best. It is usually worst at speed, when airflow through the radiator is greatest, which is why people notice it on the motorway. A weak heater in winter is more than an inconvenience, since a demister that cannot clear the screen quickly is a visibility problem.
Can I drive with a P0128 code?: Yes. This is the opposite of an overheating fault and nothing is at risk of sudden damage. What you are paying is a continuous cost: more fuel on every journey, additional engine wear from running below the designed temperature, and a heater that cannot do its job. Get it fixed at your convenience rather than treating it as an emergency, but do not leave it indefinitely, particularly with an MOT due.
Is P0128 expensive to fix?: The part is generally inexpensive — a thermostat is a small component and often comes with its housing and gasket. What varies enormously is access. On some engines it is at the end of the top hose and the job takes under an hour; on others it sits behind ancillaries or under the inlet manifold and the labour dominates. Ask for a quote that separates parts from labour before agreeing, and have the coolant level and the temperature sensor checked at the same time since both can produce this code without the thermostat being at fault.

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