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P0118: Engine Coolant Temperature (ECT) Circuit High

Quick Answer

Safe to drive (short-term)

P0118 means the engine coolant temperature (ECT) sensor circuit voltage is too high, which the ECU reads as an implausibly low temperature. Because the sensor is a thermistor whose resistance rises as it cools, an open circuit or a disconnected sensor produces a high voltage that mimics extreme cold. The ECU then over-fuels for a perpetual cold start. Repair typically costs 40 to 180 EUR for a sensor or wiring fix.

Severity
Moderate · Cooling System
Estimated repair cost
From €12 to €190+, varies by vehicle

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What does P0118 mean?

P0118 is a cooling system-related diagnostic trouble code classified as moderate severity.

What are the symptoms of P0118?

  • Check engine light illuminated
  • Temperature gauge reading cold or pinned low
  • Rich running, black smoke and a strong fuel smell
  • Poor fuel economy
  • Cooling fan not engaging when it should
  • Rough idle when the engine is actually warm

What causes P0118?

  • Coolant temperature sensor open internally (failed thermistor)

    : high

  • Disconnected or loose sensor connector

    : high

  • Open or broken signal wire to the sensor

    : medium

  • Corroded connector terminals adding resistance

    : medium

  • Open sensor ground circuit

    : low

  • ECU internal fault on the sensor input (rare)

    : low

Is it safe to drive with P0118?

Generally yes, for short-term driving. Usually safe to drive short distances. P0118 means the coolant sensor signal voltage is too high, which the ECU reads as an implausibly cold engine (an open or high-resistance circuit mimics extreme cold). This richens cold-start fuelling, can keep the cooling fan off when it is needed, and worsens fuel economy. Because the gauge may show a false low reading, confirm the real temperature in live data and fix the fault promptly so you do not miss actual overheating.

How do you diagnose P0118?

  1. Read the code and watch the live coolant temperature PID with Skanyx. P0118 typically shows the minimum value (often around -40 C) even on a warm engine, which is a clear sign of an open circuit rather than a real cold reading
  2. Touch the upper radiator hose to gauge actual temperature; if the engine is hot but the PID reads -40 C, the circuit is open
  3. Check the sensor connector first, since a disconnected or loose plug is a frequent and easy fix; reseat it and recheck the live reading
  4. Inspect the connector terminals and harness for corrosion, spread pins, or a broken signal wire causing an open circuit
  5. Measure the sensor resistance with a multimeter; an infinite or very high reading confirms an internally open thermistor
  6. If the sensor and connector are good, test the signal and ground wires for continuity back to the ECU

How much does P0118 cost to fix?

Repair Parts Cost Labor Cost Total Estimate DIY Difficulty
Replace coolant temperature sensor (DIY) €12–€45 €0–€0 €12–€45 Easy
Replace coolant temperature sensor (shop) €12–€45 €40–€100 €52–€145 Easy
Repair open wiring or connector €5–€30 €60–€160 €65–€190 Moderate
Diagnose open circuit (shop) €0–€0 €60–€120 €60–€120 Professional

Prices estimated as of October 2026. Costs vary by region, vehicle, and shop.

P0118 on specific vehicles

Toyota

Toyota Corolla

2005-2024

On the Corolla, P0118 most often traces to an open sensor or a loose connector. Check the plug is fully seated first, since a disconnected sensor is a common and quick fix before buying parts.

Volkswagen

Volkswagen Golf

2005-2024

VW Golf combined coolant sensors and their clips become brittle with heat cycling, so an open circuit from a cracked sensor body or a failed retaining clip is a common P0118 cause. The four-pin sensor is inexpensive and a frequent replacement item.

BMW

BMW 3 Series

2006-2024

On BMW 3 Series engines, P0118 can come from connector fretting or a failed sensor in the cylinder head or coolant pump housing. The high reading keeps the engine in cold-start enrichment, so over-fuelling and black smoke are typical complaints.

Ford

Ford Focus

2005-2024

The Ford Focus can set P0118 from an open sensor, a corroded connector, or a broken signal wire near the engine harness. Inspect the connector and pigtail before replacing the sensor, as wiring opens are common on higher-mileage cars.

Common questions about P0118

Why does P0118 make the gauge read cold when the engine is warm?
The coolant sensor is a thermistor whose resistance rises as it gets colder, so a high signal voltage looks identical to a very cold engine. When the circuit goes open, the voltage climbs to its maximum and the ECU interprets that as extreme cold, often around -40 C, even on a hot engine.
Can I drive with P0118?
For short trips usually yes, but the ECU thinks the engine is freezing, so it over-fuels and you get black smoke and poor economy. The fan may also stay off when it is actually needed. Confirm the real temperature in live data and fix it soon so you do not miss genuine overheating.
Could a simple unplugged connector cause P0118?
Yes, and it is one of the first things to check. A loose or disconnected sensor plug creates an open circuit, which is exactly what P0118 detects. Reseat the connector, clear the code, and re-watch the live reading before assuming the sensor itself has failed.
What is the difference between P0117 and P0118?
They are opposite faults on the same circuit. P0117 is low voltage, which mimics a hot engine and usually means a short to ground or a shorted sensor. P0118 is high voltage, which mimics a cold engine and usually means an open circuit, a disconnected plug, or a failed sensor.

Read our detailed guides

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