P0117
PowertrainEngine Coolant Temperature (ECT) Sensor Circuit Low Input
Most of the time this is a small, cheap job. The coolant temperature sensor is sending a voltage that's far too low, which the ECU reads as the engine being impossibly hot, so it sets P0117. Either the sensor itself has failed or its wiring has shorted to earth somewhere. The sensor is usually a £15 to £40 part, so the cost lives in the diagnosis and occasionally in chasing down a damaged wire.
ⓘ Information only. This page provides general educational information about fault code P0117. 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 P0117 mean?
P0117 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Engine Coolant Temperature (ECT) Sensor Circuit Low 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 the dash, sometimes with a temperature gauge pinned to the top or dead at the bottom
- • Hard starting from cold, the engine cranks longer than it should
- • Rough idle and hesitation until things warm through, then it often settles
- • Worse fuel economy because the ECU thinks it's cold and dumps in extra fuel
- • Cooling fans behaving oddly, either running flat out constantly or refusing to kick in
- • Some cars drop into limp mode and feel gutless until the fault is cleared
Possible causes
Causes commonly associated with P0117, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Failed ECT sensor, by far the most common. The internal element shorts and pulls the signal voltage to near zero
- 2. Wiring shorted to earth between the sensor and the ECU, often where the loom rubs against the block or a bracket
- 3. Corroded or water-damaged connector at the sensor, the brass pins go green and the contact breaks down
- 4. Loom chafed through by heat or vibration, common on engines where the harness runs close to the exhaust manifold
- 5. Rodent damage to the wiring, more frequent than people expect on cars left standing
- 6. ECU or PCM fault, rare, 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. Read live data and look at the ECT value. A genuine P0117 usually shows the sensor reporting an impossibly high temperature, well past 130°C, on an engine that's stone cold
- 2. Compare the ECT reading against the intake air temperature on a cold engine. Both should sit within a few degrees of each other and roughly match outside air. A big gap points at the sensor or its wiring
- 3. Unplug the connector and inspect both the pins and the sensor body for corrosion, green crust or melted plastic. This catches a fair chunk of these faults straight away
- 4. With the connector unplugged and ignition on, check you've got a steady 5-volt reference at the harness side. If the reading collapses, the short is in the loom, not the sensor
- 5. Measure the sensor's resistance cold. A healthy one reads roughly 2,000 to 3,000 ohms at around 20°C. A near-zero reading confirms a shorted sensor
- 6. Wiggle the loom and connector while watching live data, looking for the value jumping about. Clear the code and road test once you've made the repair
Common questions about P0117
What am I looking at to get this sorted, garage versus dealer? +
If it's just the sensor, an independent garage will usually have you out for under £100 all in, parts and labour, and on a lot of cars it's a ten-minute job. A main dealer will charge more for the same work, often £120 to £180 once you add their diagnostic time. The bill only climbs if the fault turns out to be a chafed or shorted wire, because that's labour to trace and repair rather than a quick part swap. Get a diagnostic done first so you're not paying for guesswork.
How do I work out whether it's the sensor or the wiring on my car? +
Start with the live data and the cold-engine comparison. Unplug the sensor and measure its resistance: if it reads near zero ohms cold, the sensor has shorted internally and that's your answer. If the sensor reads a sensible 2,000-odd ohms but the fault is still there, the problem is in the loom or the connector, so check the 5-volt reference at the harness and look for a wire shorting to earth. A green, corroded connector is the other giveaway and often the real culprit even when the sensor tests fine.
Can I just fit a new sensor myself? +
Plenty of people do, and on a straightforward installation it's a reasonable DIY job with basic tools, just make sure the engine's cold and be ready for a little coolant loss. Buy a decent branded sensor rather than the cheapest one going, the bargain ones are a known cause of repeat faults. Before you bin the old one, give the connector and the nearby wiring a proper look. If the pins are corroded or the loom is rubbed through, a new sensor on its own won't cure it.
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 →