What does engine coolant temperature high input mean?

It means the sensor's signal voltage has risen to the top of its range, which on a thermistor circuit corresponds to the coldest temperature the control unit can represent, typically around minus forty degrees. That is what an open circuit looks like electrically. A disconnected plug, a broken wire or a sensor that has failed internally are the usual causes, and a genuinely freezing engine is not.

Can P0118 make my car use more fuel?

Yes, and often dramatically. Engines run a rich mixture while cold and lean it out as they warm, and the control unit decides which map to use from the coolant temperature reading. Told the engine is permanently freezing, it stays in cold-start enrichment for every mile you drive. Drivers commonly report economy dropping by a noticeable margin, along with a rich exhaust smell and difficult hot restarts.

Will P0118 fail an MOT?

Yes, and it is one of the codes most likely to fail twice. The illuminated engine management 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. On top of that, the permanent over-fuelling raises carbon monoxide and hydrocarbon readings on a petrol car and produces smoke on a diesel, either of which can fail the emissions element on its own.

Why is my heater hot but the temperature gauge cold?

That combination is the clearest sign of a P0118. The cabin heater is fed by real engine coolant, so if it is blowing hot the engine genuinely is at temperature. The gauge is driven by the sensor reading, and the sensor is reporting an open circuit. It is worth checking the other way round too: a cold heater with a cold gauge points at a thermostat or a coolant problem instead.

Powertrain code · standard

P0118Coolant temperature sensor reading impossibly cold

Standard definition: Engine Coolant Temperature Sensor High Input

Last updated 27 July 2026

Quick answer

The coolant temperature circuit has gone high, which means the engine control unit is being told the engine is far colder than it can be, often the bottom of the scale. It normally means a broken wire or a disconnected sensor rather than a cold engine.

How the car works this out

With the sensor being a thermistor, a broken circuit sends the signal voltage to the top of its range, and the control unit reads that as the coldest temperature its table allows — commonly around minus forty. P0118 is set when that reading persists, and particularly when it contradicts the other evidence: an engine that has been running for twenty minutes cannot still be at minus forty, and the intake air temperature sensor is sitting there disagreeing.

What you might notice

Temperature gauge stuck at the cold end and never moving
Cooling fans running constantly on many cars, as a fail-safe
Heavy over-fuelling: poor economy, a rich smell, black smoke on a diesel
Hard starting when the engine is hot
Rough idle once warm, and sooty spark plugs over time
Cabin heater working normally despite the gauge saying stone cold, which is the giveaway

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 disconnected sensor plug, frequently left off after other work in that areaCommon
How to check: Look at the sensor. A plug hanging loose after a thermostat, hose or coolant change is the single most common explanation.
2A broken signal or earth wire, or a green, corroded terminal making an intermittent open circuitCommon
How to check: Bridge the two pins in the connector with a paperclip, ignition on. The reading should jump to the hottest end of the scale immediately. If it does not, the fault is in the wiring back to the control unit.
3A failed sensor gone open circuit internallyCommon
4A connector damaged by coolant weeping past the sensor sealPossible
5A sensor that has been replaced with the wrong part, so its resistance sits outside the expected tableLess common

What to do about it

1Check the plug is on. This code follows cooling system work more than any other cause, and reconnecting it costs nothing.
2Do the paperclip bridge test to separate the sensor from the wiring in under a minute.
3Note whether the cabin heater is working. A hot heater with a stone-cold gauge is close to proof that the reading is false rather than real.
4Repair it before the MOT and before you spend much more on fuel. The permanent cold reading means permanent cold-start enrichment, which is expensive at the pump and hard on the catalytic converter.
5Remember that many cars run the cooling fans continuously as a protective default when they cannot trust the temperature reading, so a flat battery can follow.

Can you keep driving?

Briefly, gently

The car will drive normally in most respects, but it is being over-fuelled the whole time. That is bad for economy, bad for the catalytic converter and, on a diesel, bad for the particulate filter, which loads up with soot faster than it can burn it off. It is not urgent in the way an overheating warning is, but it should not be left for months.

Will it fail an MOT?

The engine management lamp is a major defect and one of 371,859 recorded failures. P0118 also puts the emissions element in real jeopardy, more so than most sensor codes: the permanent cold reading keeps the engine in its warm-up enrichment, which is exactly the rich mixture the CO and hydrocarbon limits are set to catch on a petrol car, and produces visible smoke on a diesel. Clearing the code the day before the test will not help either, because the readiness monitors reset with it and the emissions check often cannot complete until they have run.

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

What does engine coolant temperature high input mean?: It means the sensor's signal voltage has risen to the top of its range, which on a thermistor circuit corresponds to the coldest temperature the control unit can represent, typically around minus forty degrees. That is what an open circuit looks like electrically. A disconnected plug, a broken wire or a sensor that has failed internally are the usual causes, and a genuinely freezing engine is not.
Can P0118 make my car use more fuel?: Yes, and often dramatically. Engines run a rich mixture while cold and lean it out as they warm, and the control unit decides which map to use from the coolant temperature reading. Told the engine is permanently freezing, it stays in cold-start enrichment for every mile you drive. Drivers commonly report economy dropping by a noticeable margin, along with a rich exhaust smell and difficult hot restarts.
Will P0118 fail an MOT?: Yes, and it is one of the codes most likely to fail twice. The illuminated engine management 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. On top of that, the permanent over-fuelling raises carbon monoxide and hydrocarbon readings on a petrol car and produces smoke on a diesel, either of which can fail the emissions element on its own.
Why is my heater hot but the temperature gauge cold?: That combination is the clearest sign of a P0118. The cabin heater is fed by real engine coolant, so if it is blowing hot the engine genuinely is at temperature. The gauge is driven by the sensor reading, and the sensor is reporting an open circuit. It is worth checking the other way round too: a cold heater with a cold gauge points at a thermostat or a coolant problem instead.

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