P0118
PowertrainEngine Coolant Temperature Sensor Circuit High Input
The ECU watches the voltage coming back from the coolant temperature sensor, and that voltage rises as the engine gets colder. When it sees the voltage pinned right up near the top of its range (basically reading as if the engine is impossibly cold or open-circuit), it logs P0118. For you that usually means a dead sensor, or more likely a broken wire or corroded plug on the sensor circuit. The engine then guesses a default temperature and runs the mixture rich, which is why it drinks fuel and idles rough until you sort it.
ⓘ Information only. This page provides general educational information about fault code P0118. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0118 mean?
P0118 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Engine Coolant Temperature Sensor Circuit High Input.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light on, sometimes with a temperature gauge that sits stone cold or reads wildly
- • Hard starting when cold, lots of cranking before it catches
- • Lumpy idle and hesitation for the first few minutes from a cold start
- • Worse fuel economy because the ECU is dumping extra fuel in
- • Cooling fans running flat out all the time, or not kicking in when they should
- • Occasional black smoke on a rich-running petrol engine
Possible causes
Causes commonly associated with P0118, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Broken, chafed or corroded wiring on the sensor circuit, this is the number one cause for a high-voltage code and points to an open circuit somewhere
- 2. Dodgy connector at the sensor, green corrosion on the pins or a plug that's worked loose with vibration and heat cycling
- 3. The ECT sensor itself failed open. Common enough on higher-mileage VAG TDIs and older Vauxhall and Ford petrols where the sensor body cracks or the element goes open
- 4. Signal wire shorted to a 5V reference somewhere in the loom, which fools the ECU into reading maximum voltage
- 5. Sensor tip uncovered by a low coolant level, so it's reading air instead of coolant and going cold
- 6. A poor earth on the sensor circuit, which shifts the voltage up just like an open would
- 7. Faulty ECM, rare, and only worth considering once everything else checks out clean
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Pull up live data and look at the reported coolant temperature with the engine cold. If it's showing something daft like minus 40 or a fixed default value while the rest of the car is at ambient, the circuit is reading wrong and you're chasing an open or a sensor failure
- 2. Check coolant level first, it takes two minutes. A low level can leave the sensor tip in air and throw the reading off
- 3. Unplug the connector and inspect the pins for corrosion, spread terminals or signs of heat. Do a wiggle test on the harness near the sensor while watching the live reading for drop-outs
- 4. With the ignition on and the sensor unplugged, back-probe for your reference voltage, usually around 5V, and confirm a clean earth. No reference voltage means the fault is in the wiring or the ECU, not the sensor
- 5. Measure the sensor's resistance cold and compare it to spec. Most read a few thousand ohms cold dropping to a couple of hundred hot. An open or hugely high reading condemns the sensor
- 6. Wiring and supply good but the sensor reads out of range? Fit a new sensor, clear the code and run it through a warm-up cycle to confirm the temperature climbs properly
Common questions about P0118
How quickly do I need to deal with this? +
Not an emergency, but don't ignore it for weeks either. The car will run and drive, just badly when cold, because it's stuck in limp mode running rich. The real concern is the fuel washing past the rings and the extra unburnt fuel cooking your catalytic converter over time. Sort it within a week or two and you'll have no lasting harm. Leave it for months and you risk a cat replacement on top of the sensor.
Is it the sensor itself or the wiring causing this? +
With a P0118 specifically (high voltage, reading too cold), the odds lean towards wiring and connectors more than a dead sensor. A high-voltage reading is exactly what you get from an open circuit, so check the plug and the loom before you spend money on parts. That said, plenty of sensors do fail open internally, especially on older engines. Test the wiring first, condemn the sensor second.
How long does the repair actually take? +
If it's just the sensor, a garage will be done in well under an hour including draining a little coolant and a road test. Budget half an hour of labour plus the part. Chasing a wiring fault is the unknown, a broken wire or corroded plug can be ten minutes or it can eat an hour of probing the loom. Most jobs come in around the £60 to £150 mark all in.
Should I buy a budget sensor or stick with genuine? +
A mid-range branded sensor from a known supplier like Bosch, Febi or Delphi is fine here and does the same job as the dealer part for a fraction of the cost. Coolant temp sensors aren't fussy. Where I'd steer clear is the £4 no-name listings, the cheap ones read a few degrees off or fail again within a year, which is a false economy when the labour is the same either way.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →