What Is P0117?
P0117 means the ECT sensor signal voltage is below the minimum expected threshold - the sensor is reporting an implausibly high temperature (ECT sensors work on a negative temperature coefficient: low voltage = high temperature reading). P0117 is stored when the ECU sees a voltage that corresponds to an engine temperature of 150°C or above - well beyond normal operating range.
In practice, P0117 usually means the sensor has failed in a low-resistance state (short circuit to earth), the wiring has shorted to earth, or the sensor connector has failed. The result is the ECU believes the engine is dangerously overheating, causing it to enter a rich protection strategy or limit fuelling. P0117 can cause significant driveability issues and may illuminate both the engine management light and the temperature warning light.
Common Symptoms
- Engine management light on
- Possible temperature warning light
- Very rich running (ECU over-fuelling as protection strategy)
- Black smoke from exhaust
- High fuel consumption
- Possible cooling fan running at maximum continuously
Common Causes
OBD Fault Code Plain English Guide
- 100 most common UK fault codes in plain English
- A severity rating for every code so you know what can wait
- Causes and next steps listed for each fault
Instant PDF download · works on phone, tablet or PC · 7-day money-back guarantee
How to Diagnose P0117
Check Coolant Level and Leaks
Before condemning the sensor, check the coolant level and look for leaks onto the wiring. Coolant on the connector is a common cause of sensor codes.
Disconnect the ECT Sensor
Unplug the ECT sensor and monitor the ECU input voltage or read the temperature on the scanner. With the sensor disconnected, the ECU should read an open circuit (very high resistance = very cold reading). If P0117 clears immediately, the sensor itself has shorted.
Test Sensor Resistance
Measure resistance across the ECT sensor pins. At 20°C it should read approximately 2,000–3,000 ohms for most UK sensors; at 80°C approximately 200–300 ohms. Near-zero resistance at ambient temperature confirms sensor failure.
Inspect Wiring for Short to Earth
With the sensor disconnected, measure resistance between the signal wire at the connector and a known earth point. It should be infinite. Any measurable resistance confirms a wiring short that must be found and repaired.
Replace ECT Sensor
If the sensor is confirmed shorted, replace it. ECT sensors are inexpensive (£10–£30) and typically straightforward to access. Drain sufficient coolant before removing to avoid a large spill.
Verdict
Disconnect the ECT sensor and check if the fault changes - this quickly confirms whether the sensor itself has shorted. If the fault is in the sensor, replacement is a cheap and simple repair. Check for coolant on the connector before assuming a sensor failure.
P0115 and P0116 - The Rest Of The Coolant Sensor Family
The engine coolant temperature sensor owns four codes, and it helps to see the ladder in one place. P0115 is the general circuit fault, P0116 means the reading is implausible for the conditions, P0117 (this page) means the signal is stuck low, and P0118 means it is stuck high. All four come from the same cheap two-wire sensor, and the tests on this page - resistance against temperature, and the live data sanity check - answer all of them.
The sanity check is the one I use first because it is free: after the car has sat overnight, the coolant temperature on live data should read within a degree or two of the outside air temperature. A sensor claiming minus 40 on a mild morning has failed low; one claiming 120 degrees on a cold engine has failed high; one that reads ambient but then jumps around while the engine warms is the classic P0116 plausibility story.
P0116 has one extra suspect the electrical codes do not: the thermostat. A stat stuck open keeps real coolant temperature so low that the ECU disbelieves the honest sensor. If P0116 arrives with slow warm-up and lukewarm cabin heat, read the P0128 guide before replacing any sensors.
