Motor Vehicle Expert
OBD fault code guide

P0118 Code Meaning UK

P0118 means “Engine Coolant Temperature Sensor High Input”. In plain English, the engine computer is seeing a coolant temperature sensor signal that is too high electrically, often caused by an open circuit, faulty sensor, damaged wiring or poor connector contact. It can make the ECU think the engine is extremely cold even when it is not.

📅 Updated June 2026 🇬🇧 UK diagnostic guide 🛠 Coolant temperature sensor advice
Quick answer Diagnosis steps Fault Codes Hub
✓ Coolant sensor fault 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 P0118 mean?

P0118 means the ECU has detected a high input signal from the engine coolant temperature sensor circuit. On many vehicles, this can happen when the circuit is open, the connector is unplugged, the wiring is damaged or the sensor has failed.

Because the ECU uses coolant temperature to control fuelling, idle speed, cooling fan strategy and emissions behaviour, P0118 can cause poor starting, rough running, high fuel use, incorrect temperature gauge behaviour or radiator fan problems.

Most common area

Coolant temperature sensor, connector, wiring, open circuit, low coolant or trapped air.

Main risk

The ECU may make bad fuelling and fan decisions because it has wrong temperature information.

Best first check

Compare live coolant temperature with the real engine temperature and inspect wiring before replacing parts.

Code meaning

P0118 — Engine coolant temperature sensor high input

The engine coolant temperature sensor tells the ECU how hot or cold the engine is. This information affects cold-start fuelling, idle speed, radiator fan operation, thermostat strategy on some vehicles and emissions control.

A “high input” code usually means the sensor circuit voltage is higher than expected. On many systems this points towards an open circuit, unplugged sensor, poor connector or failed sensor rather than the engine genuinely being too hot.

Mechanic view

Why P0118 can confuse drivers

A P0118 fault can make the car behave as if the engine is very cold. The ECU may add extra fuel, raise idle speed or run the cooling fan strangely. The dashboard gauge may also show an incorrect temperature depending on vehicle design.

This code links strongly with P0128, coolant warning light, overheating and thermostat faults. These pages help explain the surrounding cooling-system checks.

Symptoms

Common symptoms of a P0118 code

Symptoms depend on how the vehicle uses coolant temperature data and whether the sensor reading is completely wrong or intermittent.

Engine management light

A steady engine warning light is common when P0118 is stored.

Wrong temperature gauge reading

The gauge may read cold, hot or behave strangely depending on the vehicle.

Radiator fan running oddly

The cooling fan may run constantly, fail to run correctly or behave unpredictably.

Poor cold starting

Bad temperature data can affect fuelling during cold starts.

High fuel use

If the ECU thinks the engine is very cold, it may over-fuel.

Rough idle or smoke

Incorrect fuelling can cause rough running, smell of fuel or smoke on some vehicles.

Common causes

What causes P0118?

P0118 is usually caused by a coolant temperature sensor circuit issue, but cooling-system condition should also be checked.

Very common

Faulty coolant temperature sensor

The sensor may fail internally and send an incorrect signal to the ECU.

Electrical

Open circuit or damaged wiring

Broken wiring, rubbing, heat damage or previous repair work can trigger high input readings.

Connector

Corroded or loose connector

Coolant leaks, moisture or loose pins can affect the signal.

Cooling system

Low coolant or trapped air

If the sensor is not properly surrounded by coolant, readings can become unreliable.

Thermostat

Thermostat or flow issue

Cooling-system faults can appear alongside sensor codes and temperature problems.

ECU input

Reference voltage or ECU issue

Less common, but circuit testing may be needed if sensor and wiring are good.

Safe to drive?

Can you drive with P0118?

Short careful driving may be possible if the engine is definitely at a normal temperature and there is no coolant loss, warning light, overheating or poor running. However, P0118 should not be ignored because wrong coolant temperature data can affect fuelling and cooling fan operation.

✅ Lower risk: steady engine light, normal real temperature and normal running.

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

🚫 Higher risk: overheating, coolant warning light, coolant loss, steam, smoke or rough running.

Coolant Warning Light Guide Car Overheating What To Do
Overheating warning

Do not rely only on the gauge

If P0118 is active, the temperature reading may not be trustworthy. That means the dashboard gauge or warning strategy may not reflect the true engine temperature.

If you suspect overheating, stop safely, let the engine cool and check coolant level only when it is safe to do so.

Overheating Causes Overheating While Driving
UK repair costs

Typical UK repair costs for P0118

Costs vary depending on sensor access, diagnostic time, coolant condition and whether wiring repairs are needed.

Diagnostic scan and live data

Typical range: £40–£120.

Coolant temperature sensor replacement

Typical range: £50–£180+.

Wiring or connector repair

Typical range: £60–£250.

Coolant top-up and bleed

Typical range: £40–£120 depending on system design.

Thermostat replacement

Typical range: £120–£350+ if a thermostat fault is also present.

Cooling-system diagnosis

Typical range: £60–£180+ if overheating or coolant loss is present.

Repair Costs Guide UK Coolant Leak Repair Cost
Diagnosis flow

How to diagnose P0118 properly

✅ Scan all stored, pending and permanent codes.

✅ Record freeze-frame data before clearing anything.

✅ Compare live coolant temperature with actual engine temperature.

✅ Check whether the reading is stuck at an unrealistic cold or hot value.

✅ Inspect coolant temperature sensor wiring and connector condition.

✅ Check coolant level and look for trapped air or leaks.

✅ Test sensor resistance or voltage where appropriate.

✅ Check thermostat operation if P0128 or temperature issues are also present.

✅ Confirm radiator fan operation and cooling-system behaviour after repair.

Real experience

What I would check first

On a real P0118 fault, I would first look at live data. If the coolant temperature reads something unrealistic, such as extremely cold when the engine is warm, I would inspect the sensor connector and wiring before fitting parts.

If the car also has overheating symptoms, coolant loss or a coolant warning light, I would not treat it as only an electrical fault. The cooling system and sensor circuit need to be checked together.

P0128 Thermostat Code Thermostat Fault Guide Radiator Fan Not Working
MOT impact

Will P0118 fail an MOT?

P0118 itself is not normally what directly fails an MOT. The MOT concern is what the fault causes. If it causes an emissions-related engine warning light, poor running, visible smoke, overheating or emissions problems, it can become an MOT issue.

⚠️ Higher risk: engine warning light, smoke, overheating, poor running or emissions failure.

⚠️ Medium risk: high fuel use, rough idle, fan fault or related cooling-system issue.

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

Engine Light MOT Guide Common MOT Failures
Used car buying advice

P0118 on a car you want to buy

If a used car has P0118 stored, check carefully for coolant leaks, overheating history, poor starting, high fuel use and whether the temperature gauge behaves correctly.

A coolant sensor can be cheap, but overheating damage, head gasket problems or wiring faults can be more serious. Check the service history and test drive behaviour before buying.

Used Car Buying Guides Inspection Checklist
Frequently asked questions

FAQs about P0118

Common questions about P0118 coolant temperature sensor high input, safe driving, repair costs and MOT impact.

What does code P0118 mean?

P0118 means the ECU has detected a high input signal from the engine coolant temperature sensor circuit.

Can I drive with P0118?

Short careful driving may be possible if the real engine temperature is normal, but overheating, coolant loss or poor running should be checked urgently.

What is the most common cause of P0118?

A faulty coolant temperature sensor, damaged wiring, poor connector contact or open circuit are common causes.

Can P0118 cause overheating?

The code itself is a sensor circuit fault, but wrong temperature information can affect fan control and hide real cooling-system problems.

Will P0118 fail an MOT?

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

Should I replace the coolant sensor first?

Not before checking live data, wiring, connector condition, coolant level and whether the reading matches actual engine temperature.

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 P0118 guide is designed to help you understand coolant temperature sensor high 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. P0118 can involve the coolant temperature sensor, wiring, connector condition, coolant level, trapped air, fan operation and wider cooling-system behaviour, so proper testing is better than guessing.