CodeXOBD

P0118 — Engine Coolant Temperature Circuit High Input

P0118 means the coolant temperature sensor signal is stuck high, which the module interprets as an extremely cold engine. An open sensor circuit or disconnected sensor is the usual cause.

Moderate

The engine runs extremely rich because the module thinks it is permanently cold. Expect poor fuel economy, black smoke, and possible spark plug fouling. This code will cause an emissions test failure.

$10–$50Parts, typical range
0.2–1 hLabour
estimateNot a quote · 2026 basis

What P0118 means

P0118 is the opposite of P0117. The signal voltage is above the maximum threshold, meaning the thermistor resistance is too high — either open circuit or disconnected. The module reads this as an impossibly cold engine. This causes the module to run an extremely rich mixture for cold-start enrichment that never ends, leading to poor fuel economy, fouled spark plugs, and black smoke.

Technical summary
SystemPowertrain — cooling and temperature
How engine temperature is heldThe thermostat blocks flow to the radiator until the engine is warm, then opens. The coolant temperature sensor is the module's only way to know the engine's temperature, and it commands the fan from that reading. Stuck open, the engine never warms; stuck closed, it overheats; and a sensor reading wrongly produces both symptoms without either being true.Enginemakes heatThermostatopens when warmRadiatorFanwhen stationaryThe temperature sensor is the module's only sense of heat — everything follows it

Symptoms you may notice

What causes P0118

ECT sensor open-circuit

Very likelylow costeasy

The thermistor has failed open, producing infinite resistance that the module reads as extreme cold.

ECT sensor connector disconnected

Likelylow costeasy

The sensor plug has worked loose or was left off after service. An open connector produces the same signal as a failed sensor.

Signal wire broken

Possiblelow costeasy

A break in the signal wire between the sensor and the module creates an open circuit.

How to diagnose it

1

Check the sensor connector

Verify the ECT sensor connector is fully seated and the terminals are clean.

If it passes: Connector is secure. Continue to step 2.If it fails: Reconnect, clear the code, and re-test.
2

Measure sensor resistance

Disconnect the sensor and measure resistance. On a warm engine it should be low (200-500 ohms). If it reads infinite, the sensor is open.

Tools: MultimeterExpected: 200-500 ohms on a warm engineIf it passes: Resistance is in range. Check the wiring for an open.If it fails: Infinite resistance — replace the sensor.
3

Verify with live data and check wiring

With the sensor reconnected, monitor live data. Start the engine from cold — the temperature reading should climb steadily as the engine warms up. If it stays pegged cold, trace the wiring from the sensor to the module for an open.

Tools: OBD-II scanner with live data, MultimeterIf it passes: Temperature climbs normally. Fault was the sensor or connector.If it fails: Reading stays at extreme cold — inspect the wiring harness for a break.

Related codes

P0117P0125P0128

Common questions

Why does P0118 cause black smoke?

The module thinks the engine is extremely cold and applies maximum cold-start enrichment — a very rich mixture. Unburned fuel exits as black smoke.

Last verified 2026-07-27 · Sources: SAE J2012 — Diagnostic Trouble Code Definitions and Failure Type Byte (SAE International)

This information is for general guidance and is not a substitute for professional diagnosis. Cost figures are estimates, not quotes.