P0117 means the engine coolant temperature (ECT) sensor’s signal voltage is too low, which the computer reads as extremely hot coolant — whether or not the engine is actually hot. It is a sensor-circuit fault, different from P0128 (thermostat never reaching temperature) and from P0217 (a real over-temperature). Because the computer relies on this sensor for cold-start fuelling and fan control, a bad one can cause hard starting and fans that never stop.
What Does P0117 Mean?
The ECT sensor is a thermistor screwed into the coolant passages, usually near the thermostat housing. Like the intake air sensor, a hot sensor has low resistance, so a very low signal voltage is read as very hot coolant. P0117 is stored when that voltage drops below what any real engine temperature would produce — typically a short to ground or a failed sensor.
Don’t assume the engine is overheating. The code says the reading is extreme, and the reading may be false. Confirm the real temperature before acting — see our temperature warning light guide and car overheats while driving guide if the gauge or light is also telling you it is truly hot.
If the dashboard gauge or warning light also shows hot, treat it as a genuine overheat: stop safely and let the engine cool. If the gauge looks normal and the engine is not hot to the touch near the thermostat housing, the code is more likely a sensor or wiring fault.
Common Symptoms
- Check engine light on.
- Hard starting, especially when cold — the computer thinks the engine is warm and reduces cold-start fuel. See car cranks but won’t start.
- Rough running or stalling when cold — see engine runs rough only when cold.
- Cooling fans running constantly, even on a cold engine.
- A temperature gauge that reads hot, drops, or behaves erratically on cars where the gauge uses the same sensor.
- Poor fuel economy.
Common Causes, Most Likely First
- A failed ECT sensor with internal resistance too low.
- A signal wire shorted to ground or a damaged harness.
- A corroded or coolant-soaked connector.
- Low coolant leaving the sensor in air, giving odd readings — see our coolant level guide and coolant leaking under car guide.
- A rare computer fault.
How to Diagnose It
- Check the coolant level with the engine cold and top up if needed.
- Compare the scan-tool coolant temperature with reality. On a cold engine it should match the outside temperature; compare it with an infrared thermometer on the thermostat housing if you have one.
- Unplug the sensor. The reading should swing to the opposite extreme; if it does not, the wiring is shorted.
- Inspect the connector for corrosion or coolant.
- Replace the sensor if the wiring is sound. Clear the code afterwards — see our reset a check engine light guide.
Let the engine and exhaust cool before touching sensors, connectors or hoses, and never open a hot cooling system. Keep hands and loose clothing away from belts and fans, which can start on their own.
Possible Fixes
| Cause | Fix | Typical cost |
|---|---|---|
| Failed ECT sensor | Replace the sensor (partly drain coolant) | $20 – $150 |
| Shorted wiring | Repair the harness | $50 – $250 |
| Corroded connector | Clean or replace | $20 – $150 |
| Low coolant | Find the leak and refill | $20 – $500 |
Can I Drive With P0117?
Short trips only, and only after checking the engine is genuinely not overheating. The faulty reading can cause hard starting and keeps fans running, and the computer may lose proper control of the mixture. Watch the real temperature gauge and stop if it rises — see our temperature warning light guide.
When to See a Mechanic
If the code returns after the sensor is replaced, the real coolant temperature is in doubt, or the fans and gauge keep misbehaving. More in our cooling system section.
Frequently Asked Questions
What does P0117 mean?
The engine coolant temperature sensor’s signal voltage is too low, which the computer reads as extremely hot coolant.
Does P0117 mean my engine is overheating?
Not necessarily. It means the sensor circuit reports an extreme reading, which is often a sensor or wiring fault. Confirm the real temperature before acting.
How is P0117 different from P0128?
P0117 is a sensor circuit fault; P0128 means the engine is not reaching its normal operating temperature, usually because of the thermostat.
Why are my fans always on with P0117?
The computer thinks the coolant is very hot, so it runs the cooling fans to protect the engine.
How much does it cost to fix P0117?
A sensor is often $20–$150 including labour, with wiring repairs costing more.
