Motor Vehicle Expert
OBD coolant sensor fault guide

P0117 Code Meaning UK

P0117 means “Engine Coolant Temperature Sensor Circuit Low Input”. In plain English, the engine computer is seeing a coolant temperature sensor signal that is too low. On many cars this makes the ECU think the engine is extremely hot, even if it is not. Common causes include a faulty coolant temperature sensor, shorted wiring, poor connector contact, coolant contamination, low coolant or cooling system problems.

📅 Updated June 2026 🇬🇧 UK diagnostic guide 🛠 Mechanic-style cooling advice
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✓ Coolant sensor low input explained ✓ UK repair cost guide ✓ Overheating-aware advice ✓ Heavy internal linking included
Quick answer Meaning Symptoms Causes Can you drive? Repair costs Diagnosis MOT impact Related guides FAQs
Quick answer

What does P0117 mean?

P0117 means the ECU is seeing a low voltage signal from the engine coolant temperature sensor circuit. Depending on the system, a low signal can make the ECU interpret the engine as very hot.

This can affect cold starting, fuelling, radiator fan operation, emissions and overheating protection. The sensor may be faulty, but wiring, connector condition, coolant level and cooling system condition should be checked too.

Most common area

Coolant temperature sensor, wiring, connector, short to ground or coolant contamination.

Main risk

Wrong temperature data can affect fan control, fuelling, starting and overheating warnings.

Best first check

Compare live coolant temperature data with the real engine temperature when cold.

Code meaning

P0117 — Engine coolant temperature sensor circuit low input

The engine coolant temperature sensor tells the ECU how hot the engine is. The ECU uses this information for fuelling, idle speed, radiator fan control, emissions strategy and dashboard warnings.

P0117 is set when the coolant temperature sensor signal is lower than expected. This can be a sensor fault, but it can also be caused by a shorted wire, poor connector or circuit fault.

Symptoms

Common symptoms of a P0117 code

P0117 can affect starting, cooling fan control, fuelling and the temperature reading.

Engine management light

A steady engine warning light is common when P0117 is stored.

Radiator fan running constantly

The ECU may run the fan as a safety measure if it thinks the engine is hot.

Hard starting

Wrong temperature data can affect cold-start and warm-start fuelling.

Poor fuel economy

Incorrect temperature data can cause poor fuelling and higher fuel use.

Wrong temperature gauge reading

The gauge may read too hot, too cold or behave erratically depending on the car.

Cooling warning messages

Some vehicles may show coolant or temperature warnings even when coolant level is okay.

Common causes

What causes P0117?

P0117 usually points to the coolant temperature sensor signal being pulled too low.

Sensor

Faulty coolant temperature sensor

The sensor can fail internally and send an incorrect low input signal.

Electrical

Short to ground

A damaged wire can pull the sensor signal low and trigger P0117.

Connector

Corroded or wet connector

Coolant leaks or water ingress can damage the plug and affect sensor readings.

Coolant

Low coolant or air pockets

Cooling system problems can affect sensor behaviour and temperature readings.

Thermostat

Thermostat or circulation issue

Poor coolant flow can create abnormal temperature behaviour and related faults.

ECU

ECU signal issue

Less common, but ECU reference or signal interpretation problems can cause false readings.

Safe to drive?

Can you drive with P0117?

Be careful. If the engine temperature is normal and the car runs properly, short careful driving may be possible. But if the car is overheating, losing coolant, showing red temperature warnings, running badly or the fan is behaving strangely, it should be checked quickly.

✅ Lower risk: steady engine light only, normal coolant level and normal real engine temperature.

⚠️ Medium risk: fan running constantly, poor starting, wrong gauge reading or poor fuel economy.

🚫 Higher risk: overheating, coolant loss, red temperature warning, steam, misfire or severe poor running.

Coolant Warning Light Car Overheating: What To Do
Overheating warning

Do not ignore real overheating

P0117 can be an electrical fault, but you should still check whether the engine is actually overheating. A false reading and a real overheating fault can both be expensive if ignored.

Check coolant level, coolant leaks, thermostat operation, radiator fan operation and whether the heater blows hot air normally.

Overheating Causes Radiator Fan Not Working
UK repair costs

Typical UK repair costs for P0117

Costs depend on sensor access, coolant loss, wiring condition and whether a cooling system repair is also needed.

Diagnostic scan and live data

Typical range: £40–£120.

Coolant temperature sensor replacement

Typical range: £60–£220+.

Wiring or connector repair

Typical range: £60–£300+.

Coolant top-up and bleed

Typical range: £40–£120+.

Thermostat repair

Typical range: £120–£400+.

Radiator fan diagnosis

Typical range: £60–£180+ before any fan or relay repair.

Repair Costs Guide UK
Diagnosis flow

How to diagnose P0117 properly

✅ Scan all stored, pending and permanent codes.

✅ Record freeze-frame data before clearing anything.

✅ Check whether P0117 appears with P0118, overheating or fan-related faults.

✅ Read live coolant temperature data on a cold engine.

✅ Compare live data with outside temperature before starting.

✅ Inspect coolant level and check for leaks.

✅ Inspect the coolant temperature sensor plug for coolant contamination or corrosion.

✅ Check sensor wiring for rubbing, damage or short to ground.

✅ Check thermostat and fan operation if temperature behaviour is abnormal.

✅ Replace the sensor only when live data and circuit checks support it.

Real experience

What I would check first

On a real P0117 fault, I would first check live coolant temperature data before the engine is started. If the engine is stone cold but the scan tool shows an extreme hot reading, that points strongly to a sensor or circuit fault.

I would then inspect the sensor plug because coolant contamination in the connector is common on some vehicles. If the connector and wiring are clean, sensor testing or replacement becomes more sensible.

P0118 High Input P0128 Thermostat Code Diagnostic App
MOT impact

Will P0117 fail an MOT?

P0117 itself is not normally what directly fails an MOT. The concern is what the fault causes. If it causes an emissions-related engine warning light, overheating, smoke, poor running, coolant leak or unsafe behaviour, it can become an MOT issue.

⚠️ Higher risk: engine warning light, overheating, coolant leak, smoke or poor running.

⚠️ Medium risk: fan running constantly, wrong temperature reading or emissions-related running issue.

✅ Lower risk: repaired fault, no warning light and normal temperature behaviour.

Engine Light MOT Guide Common MOT Failures
Used car buying advice

P0117 on a car you want to buy

If a used car has P0117 stored, check the coolant level, temperature gauge, radiator fan behaviour, heater performance, coolant leaks and whether the code returns after clearing.

A sensor can be a simple repair, but overheating history, coolant loss or head gasket symptoms can make the risk much bigger.

Used Car Buying Guides Inspection Checklist
Frequently asked questions

FAQs about P0117

Common questions about P0117 coolant temperature sensor low input, safe driving, repair costs and MOT impact.

What does code P0117 mean?

P0117 means the ECU has detected a low input signal from the engine coolant temperature sensor circuit.

Can I drive with P0117?

Only carefully if the real engine temperature is normal. Overheating, coolant loss, red temperature warnings or poor running should be checked urgently.

Can P0117 make the fan run constantly?

Yes. If the ECU thinks the engine is too hot, it may run the cooling fan as a safety measure.

Is P0117 the same as P0118?

No. P0117 is low input, while P0118 is high input. Both relate to the coolant temperature sensor circuit.

Will P0117 fail an MOT?

It can contribute to an MOT issue if it causes an engine warning light, emissions problems, overheating, smoke, coolant leak or poor running.

Should I replace the coolant temperature sensor first?

Not before checking live data, coolant level, connector condition, wiring and whether the engine is actually overheating.

About this guide

Written for practical UK fault finding

Motor Vehicle Expert explains diagnostic trouble codes in clear, mechanic-style language for UK drivers. This P0117 guide is designed to help you understand coolant temperature sensor low input faults, likely causes, safe driving advice, repair costs and MOT implications before replacing parts.

Fault codes should always be treated as a diagnostic starting point. P0117 can involve the coolant temperature sensor, wiring, connector condition, coolant level, thermostat operation, cooling fan behaviour and real overheating symptoms, so proper testing is better than guessing.