Is P0116 the thermostat or the sensor?

More often the thermostat. P0116 is a plausibility code: the reading is electrically fine but the engine is not warming up the way the control unit expects. A thermostat stuck open lets coolant circulate through the radiator continuously, so the engine never reaches normal running temperature and the sensor reports that fact honestly. Watching the warm-up curve on live data separates them — a clean curve that stops short is the thermostat, an erratic or jumping reading is the sensor.

Will P0116 fail an MOT?

Yes. The engine management lamp is a major defect for petrol cars first used from 1 July 2003 and diesels from 1 July 2008, and 371,859 failures have been recorded against it. There is a second risk with this particular code: the emissions test is performed with the engine at normal operating temperature, and an engine that cannot reach it stays in its cold-start enrichment, which pushes the tailpipe readings up.

Why is my heater cold with a P0116 code?

Because the cause of the code is usually the same as the cause of the cold heater. The cabin heater is a small radiator fed by engine coolant, so it can only be as warm as the engine. A thermostat stuck open keeps the coolant circulating through the main radiator and the engine never gets hot enough to give you meaningful heat, particularly on short trips in winter.

Does a P0116 code waste fuel?

Yes, measurably. Engines run a deliberately rich mixture while warming up, and the control unit decides when to leave that map based on coolant temperature. An engine that never reports reaching operating temperature stays in warm-up fuelling indefinitely. Drivers often see a noticeable drop in economy, worst on short winter journeys.

Powertrain code · standard

P0116Coolant temperature sensor not warming up as it should

Standard definition: Engine Coolant Temperature Sensor Range/Performance

Last updated 27 July 2026

Quick answer

The coolant temperature sensor is working electrically, but the temperature it reports does not follow the pattern the engine control unit expects as the engine warms up. On a great many cars the sensor is innocent and the thermostat is the culprit.

How the car works this out

The control unit knows roughly how long an engine should take to reach operating temperature given the starting temperature, the running time and the load, and it cross-checks coolant temperature against intake air temperature at start-up and against oil temperature where one is fitted. P0116 is set when the reading is plausible in isolation but wrong in context — rising too slowly, never reaching normal running temperature, jumping around, or already hot on a stone-cold start. That is a very different complaint from the low and high input codes, which describe electrically impossible readings.

What you might notice

Temperature gauge slow to rise, or never reaching its usual position
Cabin heater blowing lukewarm on short journeys
Radiator cooling fan running more than it should
Poor fuel economy, because the engine stays in its warm-up fuelling map
On diesels, more smoke than usual on cold mornings

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 too early, so the engine never warms properlyCommon
How to check: Watch the coolant temperature climb on live data from cold. A reading that rises steadily then stalls short of normal running temperature, or falls again at motorway speed, is a stuck-open thermostat rather than a sensor fault.
2A sensor whose resistance curve has drifted with age, reading accurately when cold but not when hotCommon
3A low coolant level, so the sensor is sitting in air rather than liquid for part of the timePossible
How to check: Check the level cold. Erratic temperature readings that swing wildly are a classic sign of the sensor measuring a pocket of steam.
4Air trapped in the cooling system after a coolant change or a head gasket repairPossible
5The wrong sensor fitted at a previous repair, with a resistance curve that does not match the control unit's tableLess common
6A radiator cooling fan running constantly because of a separate fault, holding the engine below temperatureLess common

What to do about it

1Check the coolant level and condition before anything else. It is free, and a low or airlocked system explains a surprising share of these.
2Watch a full warm-up on live data. The shape of the curve tells you what you are dealing with: a sensor fault gives an odd or erratic reading, a stuck thermostat gives a clean curve that simply stops too low.
3Replace the thermostat if the engine is not reaching normal running temperature. It is one of the cheaper repairs on this list and it is the answer more often than the sensor.
4Fit a manufacturer-specification sensor if the sensor is at fault. Coolant temperature sensors look identical across many models and have subtly different resistance curves.
5Bleed the system properly afterwards. An airlock reintroduces exactly the symptom you have just paid to fix.

Can you keep driving?

Yes

An engine that runs a little cool is not in danger, and this is one of the more benign codes you can be handed. It costs you fuel, wears the engine slightly faster because it spends longer in warm-up enrichment, and leaves you with a poor heater in winter. The one thing to be careful of is assuming any temperature-related code is harmless: check the gauge is not reading high before you dismiss it.

Will it fail an MOT?

The lamp is the failure. An illuminated engine malfunction indicator lamp is a major defect for petrol cars first used from 1 July 2003 and diesels from 1 July 2008, with 371,859 recorded failures. P0116 carries a specific emissions trap as well: the MOT emissions test is carried out at normal running temperature, and an engine that never gets there is running on its warm-up fuelling map at the moment the probe goes in the tailpipe, which is a rich mixture by design.

The light this turns on

Engine management lightThe engine management system has found a fault it cannot correct and has stored a code for it. The car will usually still drive, but an illuminated engine light is an automatic MOT failure on petrol cars first used from 1 July 2003 and diesels from 1 July 2008.

Common questions

Is P0116 the thermostat or the sensor?: More often the thermostat. P0116 is a plausibility code: the reading is electrically fine but the engine is not warming up the way the control unit expects. A thermostat stuck open lets coolant circulate through the radiator continuously, so the engine never reaches normal running temperature and the sensor reports that fact honestly. Watching the warm-up curve on live data separates them — a clean curve that stops short is the thermostat, an erratic or jumping reading is the sensor.
Will P0116 fail an MOT?: Yes. The engine management lamp is a major defect for petrol cars first used from 1 July 2003 and diesels from 1 July 2008, and 371,859 failures have been recorded against it. There is a second risk with this particular code: the emissions test is performed with the engine at normal operating temperature, and an engine that cannot reach it stays in its cold-start enrichment, which pushes the tailpipe readings up.
Why is my heater cold with a P0116 code?: Because the cause of the code is usually the same as the cause of the cold heater. The cabin heater is a small radiator fed by engine coolant, so it can only be as warm as the engine. A thermostat stuck open keeps the coolant circulating through the main radiator and the engine never gets hot enough to give you meaningful heat, particularly on short trips in winter.
Does a P0116 code waste fuel?: Yes, measurably. Engines run a deliberately rich mixture while warming up, and the control unit decides when to leave that map based on coolant temperature. An engine that never reports reaching operating temperature stays in warm-up fuelling indefinitely. Drivers often see a noticeable drop in economy, worst on short winter journeys.

Check any vehicle's MOT history

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