Why does my temperature gauge read hot with a P0117 code when the engine is cold?

Because the gauge is showing you what the sensor is reporting, and the sensor is reporting nonsense. P0117 means the circuit voltage has fallen below its plausible floor, which on a thermistor corresponds to an extremely high temperature. A signal wire shorted to earth or a sensor that has failed internally both produce that. Confirm the engine really is cool by checking the coolant level and the top hose before you accept that explanation, because a genuinely overheating engine is a stop-driving situation.

Why are my cooling fans running all the time with P0117?

The engine control unit switches the radiator fans on according to coolant temperature, and it has been told the engine is far hotter than it is, so it runs them flat out. On some cars they carry on after the ignition is switched off, which will flatten the battery overnight. It is a symptom of the false reading, not of an overheating engine.

Will P0117 fail an MOT?

Yes. An illuminated engine malfunction indicator lamp is a major defect under section 8.2.1.2 for petrol cars first used from 1 July 2003 and 8.2.2.2 for diesels from 1 July 2008, and that defect accounts for 371,859 recorded failures. A red coolant temperature warning lamp does not have its own MOT defect, but the engine lamp alone is enough to end the test in a failure certificate.

How do I tell P0117 from a real overheating problem?

Let the engine stand until it is cold, then check the coolant level in the expansion tank and squeeze the top radiator hose. If the level is right, the hose is cool and there is no smell of coolant, the reading is false. A quicker electrical check: unplug the sensor with the ignition on, and if the gauge drops to the cold end of the scale, the wiring is intact and the sensor is the fault. If the temperature genuinely is high, stop driving and let it cool.

Powertrain code · standard

P0117Coolant temperature sensor reading impossibly hot

Standard definition: Engine Coolant Temperature Sensor Low Input

Last updated 27 July 2026

Quick answer

The coolant temperature circuit has gone to a low voltage, which on this sensor means an implausibly high temperature. The engine is almost certainly not overheating; the circuit is shorted or the sensor has failed.

How the car works this out

The coolant temperature sensor is a thermistor: hot means low resistance, which means a low signal voltage. P0117 is set when the voltage falls below the floor the control unit allows, usually a fraction of a volt, corresponding to a temperature well beyond anything a cooling system can reach. Most commonly the signal wire has shorted to earth or the sensor has failed with collapsed internal resistance. The control unit reacts as though the engine is dangerously hot, which is why the symptoms often look more dramatic than the fault is.

What you might notice

Temperature gauge pinned at maximum, sometimes with a red coolant warning light
Radiator cooling fans running constantly, including with the engine cold and the ignition off on some cars
Hard starting from cold, because the control unit withholds the cold-start enrichment
Rough running and hesitation when cold
Reduced power or limp mode on cars that protect themselves against overheating
Battery going flat overnight where the fans keep running

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.

1The signal wire shorted to earth, often where the loom rubs on the cylinder head or a bracketCommon
How to check: Unplug the sensor with the ignition on. The reading should swing to the coldest end of the scale. If it stays pinned at maximum temperature with the sensor disconnected, the wiring is shorted.
2A failed sensor with internally collapsed resistanceCommon
3Coolant or water inside the connector bridging the terminalsPossible
How to check: Unplug and look for crusty coloured deposits, which mean coolant has been wicking up the wires from a weeping sensor seal.
4A damaged connector with pins pushed together or a bent terminalPossible
5A genuinely overheating engine, which is uncommon with this code but must be ruled outLess common
How to check: Feel the top hose and the heater output, and check the coolant level cold. If the engine really is hot, stop driving it — that is a different and far more serious problem.

What to do about it

1Satisfy yourself first that the engine is not actually overheating. Check the coolant level when cold and whether the top hose is genuinely hot. Everything else on this list assumes a false reading.
2Unplug the sensor with the ignition on and watch the reading. Swinging to the cold extreme means the wiring is sound and the sensor is at fault; staying pinned hot means the signal wire is shorted to earth.
3Inspect the connector for coolant contamination and the loom for chafe points against the head or the manifold.
4Fix it promptly even though the engine is fine. Constantly running cooling fans flatten batteries, and a car that will not enrich for a cold start becomes very unpleasant on winter mornings.

Can you keep driving?

Only to a garage

Once you have established the engine is not genuinely hot, the car is safe to drive but unpleasant and self-defeating: cold starts are poor, the fans run flat out, and some cars limit power to protect an engine they think is overheating. The bigger risk is complacency — if you get used to ignoring a maxed-out temperature gauge, you will ignore it when it is real.

Will it fail an MOT?

Two separate MOT problems. The 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 against it. On top of that, the emissions test needs the engine at normal operating temperature and the tester relies on the car's own reading; a car reporting a false over-temperature may also have the cooling fans running hard enough to make holding a steady fast idle awkward.

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

Why does my temperature gauge read hot with a P0117 code when the engine is cold?: Because the gauge is showing you what the sensor is reporting, and the sensor is reporting nonsense. P0117 means the circuit voltage has fallen below its plausible floor, which on a thermistor corresponds to an extremely high temperature. A signal wire shorted to earth or a sensor that has failed internally both produce that. Confirm the engine really is cool by checking the coolant level and the top hose before you accept that explanation, because a genuinely overheating engine is a stop-driving situation.
Why are my cooling fans running all the time with P0117?: The engine control unit switches the radiator fans on according to coolant temperature, and it has been told the engine is far hotter than it is, so it runs them flat out. On some cars they carry on after the ignition is switched off, which will flatten the battery overnight. It is a symptom of the false reading, not of an overheating engine.
Will P0117 fail an MOT?: Yes. An illuminated engine malfunction indicator lamp is a major defect under section 8.2.1.2 for petrol cars first used from 1 July 2003 and 8.2.2.2 for diesels from 1 July 2008, and that defect accounts for 371,859 recorded failures. A red coolant temperature warning lamp does not have its own MOT defect, but the engine lamp alone is enough to end the test in a failure certificate.
How do I tell P0117 from a real overheating problem?: Let the engine stand until it is cold, then check the coolant level in the expansion tank and squeeze the top radiator hose. If the level is right, the hose is cool and there is no smell of coolant, the reading is false. A quicker electrical check: unplug the sensor with the ignition on, and if the gauge drops to the cold end of the scale, the wiring is intact and the sensor is the fault. If the temperature genuinely is high, stop driving and let it cool.

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