⚠ P0118 MEDIUM SEVERITY

P0118: what it means

Officially? A wall of jargon. To most people? Absolutely nothing. That's exactly why we built the app.

The official definition vs. an actual explanation

Plug into any code reader and you'll get the same line every other app shows you. Technically correct. Practically useless.

What other apps tell you

"P0118: The coolant temperature sensor signal is stuck high. The ECU sees an impossibly cold reading, pointing to a wiring or sensor fault."

What OBD Mechanic tells you

"Same family of fault as P0113, but on the engine coolant temperature sensor: the signal voltage is stuck high, which the ECU reads as an absurdly cold engine, minus 40 territory. That's electrics, not temperature: a dodgy connector, a broken wire, or the sensor gone open-circuit. The knock-on is real though. An ECU that thinks the engine is freezing will over-fuel it (running rich, smelly exhaust, poor economy) and the cooling fan may run flat out as a precaution. Cheap sensor, usually an easy job."

What actually causes it, cheap to dear

Worth checking in roughly this order. Don't jump straight to the expensive fix:

Corroded, loose or unplugged connector at the coolant temp sensor
Broken or chafed signal wire between sensor and ECU
Failed coolant temperature sensor (open circuit)
Rarely: an ECU input fault, genuinely last on the list

usually cheap depends pricey

Suggested checks

Find the sensor. It's typically threaded into the thermostat housing or cylinder head, near where the top radiator hose meets the engine. Check the plug is secure and the pins clean
Wiggle-test the wiring with ignition on while watching the coolant temp reading for jumps
Sanity-check the reading: stone cold overnight it should match outside temperature, not -40
If wiring's good, replace the sensor. It's cheap, though you'll lose a little coolant, so have some ready to top up

Which dials to watch for P0118

The code itself is a snapshot. The fault sets after the car's watched the numbers over a whole drive cycle. Here's what's worth keeping an eye on:

Coolant temp

Pegged at -40°C, or dropping off a cliff mid-drive? That's the circuit fault in action. A real engine warms smoothly to about 90°C and stays there.

Short fuel trim

An ECU that believes the engine's freezing dumps in extra fuel. Watching the trims shows you how much the bad reading is skewing the fuelling.

Not simulated live Our free demo drive doesn't model this reading yet. Watch it live on your own car in the app.

Live simulation on the readings our free demo drive supports. This is how the app’s dials move.

It spots things you'd never see in the numbers

Codes rarely tell the whole story. The clever part is reading them together, and against your exact car. Here's the kind of thing it catches:

AI Mechanic

What our mechanic says

P0118

Same family of fault as P0113, but on the engine coolant temperature sensor: the signal voltage is stuck high, which the ECU reads as an absurdly cold engine, minus 40 territory. That's electrics, not temperature: a dodgy connector, a broken wire, or the sensor gone open-circuit. The knock-on is real though. An ECU that thinks the engine is freezing will over-fuel it (running rich, smelly exhaust, poor economy) and the cooling fan may run flat out as a precaution. Cheap sensor, usually an easy job.

P0118: quick questions

Can I still drive it?

For short hops, yes, but there are two catches. The temperature gauge and the cooling fan rely on this sensor, so with it lying, neither you nor the ECU can spot genuine overheating. If you see steam or a red temperature light, stop when safe. The engine also runs rich while the ECU thinks it's cold, which wastes fuel and isn't kind to the catalytic converter. Fix it soon rather than someday.

How much will it cost to fix?

The sensor itself is usually £10-30, and on most engines it's accessible enough that a garage will do it inside an hour's labour. DIY is realistic if you're comfortable losing and topping up a bit of coolant. Wiring repairs, if that's the cause, are similarly modest.

Will it pass the MOT?

A lit engine management light has been an MOT fail item in the UK since 2018, so fix the fault first. The rich running this causes can also push up emissions, which the tailpipe test measures. It's a cheap fix, so sort it before the test.