Engine

P0118 — Engine Coolant Temperature Sensor Circuit High Input

Severity: medium Drive with care, to a shop

What P0118 means

The engine computer sends a 5-volt reference to the coolant temperature sensor, whose resistance drops as the engine warms up. P0118 sets when the signal voltage stays above the expected range, which the computer reads as an impossibly cold engine (around -40°F/°C). It is usually caused by an open circuit or a bad sensor rather than a real temperature problem.

Symptoms

  • Check Engine light on
  • Poor fuel economy
  • Hard starting, especially when warm
  • Rough idle or misfires
  • Cooling fan running all the time
  • Temperature gauge reads cold or pegged
  • Black exhaust smoke from a rich mixture

Possible causes

  1. Failed ECT sensor
  2. Open in the signal or ground wire to the sensor
  3. Corroded or damaged sensor connector
  4. Signal wire shorted to voltage
  5. Low coolant leaving the sensor exposed to air
  6. Stuck thermostat or true overheating (less common cause of this code)
  7. Faulty PCM (very rare)

How to diagnose

  1. Scan for codes and view live data: with a cold engine, coolant temperature should be near ambient.
  2. A reading near -40° points to an open circuit or lost connection.
  3. Check coolant level and condition, and top up if low.
  4. Inspect the sensor connector and harness for corrosion, melted insulation, chafing or water.
  5. Unplug the sensor and jumper the signal terminal to ground: the scan tool should jump to a very high temperature, proving the PCM and signal wire are good.
  6. Use a multimeter to check for 5V reference and a good ground at the connector.
  7. Measure sensor resistance and compare it to the spec chart for your engine at a known temperature.
  8. Compare scan tool data with an infrared thermometer reading at the thermostat housing.

Common fixes

  • Replace the ECT sensor
  • Repair the wiring, terminals or connector
  • Top up or replace coolant and bleed the system
  • Replace the thermostat if the engine does not warm up or overheats
  • Replace or reprogram the PCM only after everything else is ruled out

Questions and answers

What does P0118 mean?

The PCM is getting too high a voltage from the coolant temperature sensor circuit, so it thinks the engine is extremely cold. The cause is usually an open circuit or a bad sensor.

Can I drive with code P0118?

Briefly and with care, yes, but watch the temperature gauge. Bad data can make the engine run rich and keep the fan running nonstop.

How do I test the coolant temperature sensor?

Compare scan tool temperature with the actual temperature of a cold engine and measure the sensor's resistance. Grounding the signal wire at the unplugged connector shows whether the wiring and PCM are okay.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.