P0117 — Engine Coolant Temperature Sensor Low Input

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

Failed ECT Sensor (Short to Earth)The sensor resistance element has shorted internally, creating near-zero resistance regardless of actual temperature. This produces a voltage reading below the minimum valid threshold.

Signal Wire Shorted to EarthThe sensor signal wire has shorted to the vehicle earth in the wiring loom or at the connector, pulling the ECU input to near zero volts.

Sensor Connector ShortMoisture or corrosion bridging the signal pin to the earth pin inside the connector creates the same effect as a wiring short.

Coolant Leaks Causing Electrical ShortsA coolant leak onto the ECT connector can cause short circuits between pins, producing false low-voltage readings.

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.

Frequently Asked Questions

When the ECU believes the engine is overheating (low ECT voltage = very high temperature reading), it may enrich the mixture as an engine protection strategy, or simply because its calibration tables apply incorrect fuel corrections for the false temperature reading.

Yes. If the ECT sensor is also connected to the dashboard temperature gauge (many vehicles use a separate sender), a shorted sensor can cause the gauge to read maximum, triggering driver concern.

In a genuine overheating situation, the coolant temperature really is above 110°C. With P0117, the actual engine temperature is typically normal — only the sensor is faulty. Confirm actual temperature by feeling hoses carefully or using an infrared thermometer.

The engine is likely running richer than intended, which is tolerable for short distances. However, you cannot trust the temperature gauge or warning system, meaning a genuine overheat might go undetected. Repair promptly.

It depends on which warning light the code is triggering. Since 2018, any car presenting with an illuminated amber Engine Management Light (EML) at the MOT is a Major failure under DVSA rules — even if the car drives perfectly. A red warning light is always a Major or Dangerous failure depending on context. If clearing the fault makes the light go out and the code does not reappear during the pre-test drive, you will pass; if the code returns within minutes of clearing, the underlying fault must be fixed before MOT day. A tester is required to fail the car on the light being on, regardless of whether the underlying fault is something safety-critical or not. For codes that affect emissions specifically (catalyst, lambda, EGR), the car may also fail the actual emissions check. Fix the cause, clear the code, and drive the car for a few miles before the test.