Cooling

P0117 — Engine Coolant Temperature Sensor Circuit Low Input

Severity: medium Drive with care, to a shop

What P0117 means

The PCM sends a reference voltage to the coolant temperature sensor and reads the voltage drop to work out engine temperature. P0117 sets when the signal voltage stays below the expected range, which the PCM reads as an impossibly hot engine. The usual culprits are a failed sensor or a signal wire shorted to ground.

Symptoms

  • Check Engine light on
  • Poor fuel economy
  • Rough idle and poor drivability
  • Hard starting, especially when cold
  • Black exhaust smoke from a rich mixture
  • Cooling fan running constantly or at the wrong time
  • Temperature gauge reading incorrectly or not moving

Possible causes

  1. Failed coolant temperature sensor
  2. Signal wire shorted to ground
  3. Damaged or chafed wiring harness
  4. Corroded connector or terminals
  5. Moisture in the sensor connector
  6. Faulty PCM (rare)

How to diagnose

  1. Read codes and live data, and compare the scan tool coolant temperature with actual temperature on a cold engine.
  2. If the scanner shows near-maximum (about 265-285°F) on a cold engine, suspect a short to ground or a bad sensor.
  3. Inspect the sensor connector and harness for corrosion, moisture and chafed wires.
  4. Unplug the sensor: if the reading drops to the minimum (about -40°), the sensor is faulty.
  5. Measure the sensor resistance with a multimeter and compare it to the spec for your engine.
  6. Check the signal wire for a short to ground between the sensor and PCM.
  7. Verify the roughly 5 V reference voltage at the connector with the key on.
  8. Check the coolant level and make sure there are no air pockets in the system.

Common fixes

  • Replace the coolant temperature sensor
  • Repair or replace damaged wiring
  • Clean or replace the connector and terminals
  • Top up coolant and bleed the cooling system
  • Diagnose and, as a last resort, replace the PCM

Questions and answers

What does P0117 mean?

The PCM is getting a very low voltage from the coolant temperature sensor, so it thinks the engine is extremely hot. It is usually a bad sensor or a wire shorted to ground.

Can I drive with P0117?

Only for a short time and with care. The PCM is working from wrong data, so you may see poor mileage, hard starting or overheating. Fix it soon.

How do I tell a bad sensor from a wiring problem?

Unplug the sensor with the key on. If the scan tool temperature drops to the minimum, the wiring to the PCM is fine and the sensor is bad. If the code stays, look for a short in the harness.

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.