P0118 — Engine Coolant Temperature Sensor High Input

What Is P0118?

P0118 means the ECT sensor signal voltage is above the maximum expected threshold — the sensor is reporting an implausibly low temperature (high voltage on a NTC sensor = cold reading). The ECU reads a voltage corresponding to a temperature below -40°C or at an extreme cold value, which is clearly impossible on a running engine.

P0118 typically means the sensor has failed open-circuit, the signal wire has broken or lost its earth reference, or the connector has failed. The ECU believes the engine is permanently cold, so it applies continuous cold-start enrichment — running rich on every journey regardless of actual engine temperature. This increases fuel consumption, fouls spark plugs, and damages the catalytic converter over time.

Common Symptoms

Engine management light on
High fuel consumption on every trip
Fuel smell from exhaust
Possible black smoke
Fouled spark plugs
Cooling fans may not activate correctly
Rough or hunting idle as engine warms up

Common Causes

Failed ECT Sensor (Open Circuit)The sensor resistance element has broken internally, creating infinite resistance — which the ECU reads as a maximum voltage (very cold reading).

Broken Signal or Earth WireA break in the signal wire causes the ECU input to float high (reading minimum temperature). A broken earth wire has a similar effect.

Corroded or Damaged ConnectorSevere corrosion in the connector breaks the electrical path between the sensor and ECU, causing an open-circuit condition.

Sensor Pulled Out of HousingOn vehicles where the ECT sensor is threaded into the thermostat housing, a sensor that has worked loose partially loses thermal contact and electrical continuity.

How to Diagnose P0118

Disconnect ECT Sensor and Check Reading Change

With the sensor disconnected, the ECU should show an open-circuit high reading (maximum cold). If the scanner already shows maximum cold and the sensor is connected, disconnecting it should produce no change — confirming an open circuit in the sensor or wiring.

Test Sensor Resistance

Measure resistance across the sensor pins at ambient temperature (approximately 20°C). Should read 2,000–3,000 ohms. Infinite resistance (open circuit) at ambient confirms sensor failure.

Check Wiring Continuity

Check for continuity of the signal and earth wires from sensor to ECU. A broken wire will show no continuity. Repair any broken wires before replacing the sensor.

Inspect Connector

Check for severely corroded pins inside the connector. Corrosion can build to the point of breaking the electrical connection. Terminals may need replacement rather than just cleaning.

Replace ECT Sensor

ECT sensors are inexpensive (£10–£30). Drain coolant as needed, replace the sensor with a new unit, refill and bleed the cooling system, then clear the code and verify the temperature reading rises correctly on the scanner.

Verdict

P0118 causes constant rich running — every journey sees excess fuel enrichment because the ECU thinks the engine is always cold. An open-circuit ECT sensor is the most common cause and is cheap to replace. Fix promptly to avoid catalytic converter damage from sustained rich running.

Frequently Asked Questions

Yes. Continuous cold-start enrichment sends excess fuel into the exhaust. Unburnt fuel in the catalytic converter causes it to overheat during combustion of that fuel, and sustained rich running eventually destroys the catalyst internally.

Test sensor resistance directly across the sensor pins. If infinite (open circuit), it is the sensor. If resistance is normal (2,000–3,000 ohm at ambient), the fault is in the wiring or connector between the sensor and ECU.

Possibly — depends on whether the gauge uses the same sensor or a separate sender. If the same sensor, the gauge may read maximum cold even with the engine fully warm.

The code will return immediately because the underlying fault persists. The engine will continue running rich. Repair rather than clear.

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.