P0117: Engine Coolant Temperature Sensor 1 Circuit Low
The coolant temperature sensor voltage is too low, so the computer is reading the engine as extremely hot.
What P0117 means
With P0117, the engine computer sees the coolant temperature signal stuck below its normal range, usually under about 0.2 to 0.3 volts. Because the sensor voltage drops as the engine warms up, a very low voltage reads as an extremely hot engine. That makes the computer lean out the mixture, turn on the cooling fans and sometimes reduce power to protect the engine. A cold engine reading as hot is hard to start because it does not get the extra fuel it needs. RepairPal lists a defective sensor, corroded wiring and rusty coolant as common triggers. Before replacing anything, a shop confirms whether the engine is really hot. If it is not, they unplug the sensor and watch the reading. If the reading jumps to very cold, the sensor was shorted. If it stays low, the wiring is shorted to ground. Fixing it is usually simple, but a real overheating engine needs attention right away.
Common symptoms
- Check engine light on
- Cooling fans running constantly
- Hard starting when cold
- Poor gas mileage
- Temperature gauge pegged hot on some cars
Most common causes
- Shorted ECT sensor
- Sensor signal wire shorted to ground
- Damaged or corroded connector
- Engine actually overheating
- Rusty coolant affecting the sensor
What to check yourself first
First confirm the engine is not truly overheating: feel the upper radiator hose carefully and check coolant level when cool. Inspect the ECT connector and wiring for damage. Unplugging the sensor should make the scan tool reading jump to very cold, which suggests the sensor was shorted.
Typical fixes and what they cost
| Fix | Typical cost | Note |
|---|---|---|
| Replace engine coolant temperature sensor | $499-$585 | The most common repair once wiring is checked. |
| Electrical diagnosis to find a wiring or connector fault | $122-$179 | For a wire rubbed through to ground or a damaged connector. |
| Coolant drain and refill | $440-$512 | Recommended if the coolant is rusty or overdue. |
Typical US price ranges for parts and labor, not quotes. Your shop should confirm the cause with a diagnostic before replacing parts.
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Related codes
- P0118 Engine Coolant Temperature Sensor 1 Circuit High
- P0115 Engine Coolant Temperature Sensor 1 Circuit
- P0116 Engine Coolant Temperature Sensor 1 Circuit Range/Performance
- P0480 Fan 1 Control Circuit
- P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)