Quick answer: The P0117 code indicates that the Engine Coolant Temperature (ECT) sensor circuit has a low voltage output, causing the engine computer to read an unreasonably high coolant temperature. This directly impacts the engine’s cooling and fuel management systems, making prompt diagnosis critical to avoid engine damage.
Seeing a check engine light combined with potential cooling system alerts can make any driver uneasy. Code P0117 means your car’s main computer thinks the engine is running extremely hot, even when you have just started it on a cold morning. In this guide, we will break down what triggers this code, how to perform basic checks yourself, and how to fix the underlying problem.
What Does P0117 Mean?
The Engine Coolant Temperature (ECT) sensor is a thermistor that changes its electrical resistance based on the coolant temperature. The engine control module supplies a 5-volt reference signal to the sensor. As the engine warms up, the sensor’s resistance drops, which lowers the return signal voltage. When code P0117 is stored, the computer has detected a signal voltage below 0.2 volts for longer than a few seconds. To the computer, this extremely low voltage signal means the coolant temperature is artificially high, often reading over 250°F (120°C).
Symptoms
- Illuminated check engine light on the dashboard.
- Radiator cooling fans running continuously at high speed.
- Temperature gauge reading hot or pinned to the maximum mark.
- Engine hard starting or rough idling when cold.
- Poor fuel economy caused by incorrect fuel injection calculations.
- Engine entering limp mode or shutting down to prevent perceived overheating.
Possible Causes
A common mistake is automatically replacing the temperature sensor, whereas damaged electrical wiring or poor connections are equally frequent culprits.
- Short to ground in the ECT sensor signal wire or damaged harness.
- Faulty Engine Coolant Temperature (ECT) sensor.
- Corroded, loose, or damaged sensor connector pins.
- Extremely low coolant level or actual engine overheating condition.
- Stuck open thermostat keeping the engine from reaching operating temperature correctly.
- Failed Powertrain Control Module (PCM), though this is very rare.
Check These Yourself Before Visiting a Mechanic
- Check the engine coolant level in the expansion tank (only when the engine is fully cooled down) to confirm the engine is not actually low on fluid.
- Visually inspect the wiring leading to the ECT sensor for obvious signs of burning, chafing, or animal bites.
- Unplug the connector at the coolant sensor and check the internal metal pins for rust, green corrosion, or moisture buildup.
How to Fix It
- Connect an OBD-II scanner to view live sensor data and check the engine coolant temperature reading.
- Inspect the wiring harness and electrical plug at the ECT sensor for ground shorts or pin damage.
- Disconnect the ECT sensor and test its internal resistance with a digital multimeter, comparing the reading to factory technical specs.
- Check for 5 volts of reference voltage and proper ground at the harness connector using a multimeter.
- Replace the Engine Coolant Temperature sensor if its internal resistance is out of range or shorted.
- Repair any shorted or damaged wiring in the sensor circuit before clearing the trouble code.
Estimated Repair Costs (2026)
| Job | Parts ($) | Labor ($) |
|---|---|---|
| ECT Sensor Replacement | $20 – $60 | $80 – $150 |
| Wiring Repair / Harness Service | $10 – $30 | $100 – $200 |
| Coolant Flush & Refill | $20 – $40 | $70 – $120 |
| Thermostat Replacement | $30 – $70 | $100 – $180 |
Note: these are 2026 estimates in US dollars; costs vary by make and model.
Frequently Asked Questions
Can I drive with the P0117 code?
It is not advisable to drive with a P0117 code. If the engine is genuinely overheating, continuing to drive will cause severe damage such as blown head gaskets or a warped cylinder head. If it is an electrical glitch, the vehicle may run rich, stall, or enter a low-power limp mode.
How do I clear the P0117 code?
You can clear the code using an OBD-II diagnostic scanner after resolving the underlying electrical or mechanical issue. Once the computer detects a normal voltage signal from the sensor during a drive cycle, the code will clear and the check engine light will turn off.
Where is the Engine Coolant Temperature sensor located?
The sensor is usually threaded into the engine block, cylinder head, or thermostat housing along the path of flowing engine coolant. Check your vehicle’s specific repair manual, as some modern cars feature multiple temperature sensors across the cooling system.