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P0117Engine coolant temperature sensor circuit low input

Content reviewed: Content reviewed by a heavy-duty technician. About our editorial approach

Check soon

— The engine may not be overheating at all, but the wrong signal makes the ECM fuel and cool it incorrectly — fix it before chasing phantom cooling problems.

What does this severity mean?

General guidance only — not a substitute for a professional inspection or official roadside / out-of-service criteria (e.g. CVSA OOSC) in your jurisdiction.

Part of system: Engine cooling & overheat protection

Means the coolant temperature signal voltage fell below the low threshold, which the ECM interprets as an impossibly hot engine because these sensors are negative-temperature-coefficient thermistors whose resistance and signal voltage drop as heat rises. In practice the cause is usually electrical rather than thermal: a signal wire shorted to ground, a harness chafed against the cylinder head, or a sensor that failed short internally. Compare the reported value with an infrared reading on the thermostat housing, and on a fully cold engine check that coolant and intake air temperature agree within a few degrees. Unplugging the sensor should send the reading to the opposite extreme; if nothing changes, the short is in the harness rather than the sensor. A false hot reading makes the ECM lean the mixture and run fans continuously.

Parts that may be involved

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Vehicles often affected

Sensors mounted low in the thermostat housing collect corrosion, and harnesses routed across the head are the usual chafe point on older engines.

Typical symptoms

  • Temperature gauge pinned hot while the engine is still cold
  • Cooling fans running constantly from key-on
  • Very lean cold-start air–fuel mixture (too much air vs fuel, ~14.7:1 target) with hesitation
  • Coolant reading far above intake air on a cold engine
  • Hard start because cold enrichment is cancelled

Likely causes

  • Signal wire shorted to ground along the harness run
  • Sensor thermistor failed short internally
  • Connector wet or corroded, bridging signal to ground
  • Harness chafed against the cylinder head or exhaust
  • Wrong sensor fitted with an incorrect resistance curve

Recommended checks

  1. 1

    Compare the reading to an infrared measurement at the housing

  2. 2

    Check that coolant and intake air agree on a cold engine

  3. 3

    Unplug the sensor and confirm the reading swings to the other extreme

  4. 4

    Inspect the harness for chafe and the connector for moisture

  5. 5

    Fit the correct part number and retest at operating temperature

DIY vs professional

DIY: sensor replacement and connector cleaning on most engines. Pro: harness repairs inside the loom and resistance-curve verification.

See also

Related terms

Content reviewed by a heavy-duty technician. About our editorial approach