P0117: Engine Coolant Temperature Sensor Circuit Low
P0117 means the engine computer sees a signal voltage from the coolant temperature sensor that is too low, as if the engine were extremely hot. It is usually not an emergency, but the computer is working with bad temperature data, so have it fixed soon.
You can drive, but get it checked soon
The car may run, but the computer believes the engine is hot, so cold starts can be hard and the radiator fan may run constantly. The limp mode (reduced-performance backup operation) is possible. Because the real coolant temperature is not being reported correctly, the gauge and the fan may not warn you about actual overheating, so keep an eye on the engine and repair it soon.
What it means
The coolant temperature sensor is a thermistor, a resistor whose resistance changes with temperature. The engine control module sends it 5 volts through a resistor, and as the coolant warms up, the sensor's resistance and the signal voltage drop. The module uses this reading for extra fuel on cold starts, idle speed, ignition timing, the radiator fan, and for deciding when to switch to closed-loop mixture control (fine-tuning fuel using the exhaust oxygen sensors). P0117 sets when the signal voltage falls too low, which the module reads as a coolant temperature of roughly 140 to 160 °C, an implausible value. Typical triggers are a signal wire shorted to ground, coolant or moisture in the connector, or a sensor that has shorted internally. Exact limits differ by manufacturer, and some require the fault to persist for a few seconds or on two trips. Some Ford engines measure cylinder head temperature instead and calculate coolant temperature from it. On those, the equivalent code is P1290.
Likely causes
- Signal wire shorted to ground (chafing, pinched harness)Very likely
A wire that touches metal pulls the signal voltage to zero, which the module reads as a very hot engine.
- Sensor shorted internally or coolant inside the sensorVery likely
A failed sensor or one with coolant inside can drop its resistance and pull the signal voltage too low.
- Moisture or coolant in the connectorPossible
Liquid in the plug can create a path between the terminals or to ground and lower the voltage.
- Engine control module faultyLess likely
A fault in the module's sensor input can cause a wrong reading, but this is the least likely cause.
What you may notice
- Check engine light on
- Radiator fan runs constantly
- Hard cold start (no extra fuel, because the engine is reported as hot)
- Temperature gauge at the hot end
- A/C may be switched off
What to check, cheapest first
- 1Look at the sensor connector for moisture, coolant, corrosion, or a loose fit, and dry or reseat it. · you can do it
- 2Inspect the sensor wiring for chafing, pinched spots, or damage where it touches metal or runs near hot or moving parts. · you can do it
- 3Check for coolant leaking at or inside the sensor, and make sure the coolant level is correct on a cold engine. · you can do it
- 4With a scan tool, read the coolant temperature value on a cold engine and see if it shows an extreme reading that doesn't match the real temperature. · mechanic
- 5Unplug the sensor. If the reading changes to a very cold value, the sensor is likely at fault. If it stays low, test the wiring for a short to ground. · mechanic
- 6Replace the sensor if it is faulty, clear the code, and complete a drive cycle to confirm it does not return. Test the control module only if the sensor and wiring check out. · mechanic
Related codes
Frequently asked questions
Can I drive with a P0117 code?+
Short trips are usually possible, but starting may be hard and the fan may run constantly. Since the computer isn't getting a real temperature, have it repaired soon and watch for signs of true overheating.
Can a loose gas cap cause P0117?+
No. P0117 is about the coolant temperature sensor circuit. A gas cap problem sets different codes.
Will P0117 pass an emissions test?+
Unlikely. The check engine light is on and the code is emissions-related, so the car would normally fail until it is repaired and the system has completed its drive cycle checks.
Is P0117 expensive to fix?+
It is rated an easy repair that many owners can do themselves, taking roughly 0.3 to 1.5 hours. Cost depends on whether it is a sensor, a connector, or wiring.
Written with the help of AI from the public OBDex definition (CC0). Each make may use the code differently. Updated: 2026-11-10