P0117 indicates the Engine Coolant Temperature (ECT) sensor circuit is reporting a lower-than-expected input to the engine control module (ECM/PCM). In practice, this is most often caused by a faulty sensor, damaged wiring or a bad connector, but it can also be triggered by issues in the ECU or related circuits. This article explains what P0117 means, common causes, diagnostic steps, and fixes used by technicians.
What P0117 Means
Engine coolant temperature sensors tell the PCM how hot or cold the engine is. If the PCM sees a low input value when the engine is at running temperature or a temperature that doesn’t align with reality, it flags P0117. This can affect fuel trim, idle, and overall drivability, especially during cold starts or when the engine is warming up.
Common causes
The following are the typical reasons P0117 appears. A visual check plus a few tests usually identify the root cause.
- Faulty coolant temperature sensor (ECT)
- Damaged, frayed, or corroded wiring or a loose/dirty connector
- Short to ground or to the reference voltage on the sensor circuit
- Poor ECU/PCM input or a faulty PCM
- Low coolant level or a thermostat issue causing abnormal engine temperatures
If any of these conditions are present, addressing the sensor, wiring, or thermostat can resolve P0117.
Diagnostic steps
Use this step-by-step approach to confirm the cause and verify a fix. Start with the simplest visual checks, then proceed to electrical testing and, finally, component replacement if needed.
- Verify the DTC with a scanner and review freeze-frame data to see the reported coolant temperature and sensor voltage.
- Visually inspect the ECT sensor and wiring harness for damage, oil contamination, corrosion, or loose connections at the sensor and PCM connector.
- Check the coolant level and condition. Ensure there is adequate coolant and that the system is not leaking, which could affect readings.
- Test the sensor resistance at different temperatures or use live data to compare the sensor reading with actual coolant temperature. Refer to the vehicle’s service manual for the correct resistance values vs. temperature.
- Inspect the sensor power and ground circuits: verify the 5V reference (from the PCM) and the ground circuit with a multimeter. Look for a short to ground, short to 5V, or high resistance in the wiring.
- Inspect the harness for chafed wires near moving parts (engine heat, belt areas, near the thermostat housing), and repair as needed. Reconnect and secure all connectors properly.
- Consider thermostat or cooling system issues. If the thermostat is stuck open, the engine may run cooler than expected and confuse readings; replace thermostat if needed and retest.
- Clear codes and perform a road test to confirm the code does not return and that live data show the ECT sensor reporting a realistic temperature corresponding to engine temperature.
After these steps, you will likely identify the issue as a faulty sensor or a wiring fault; if those are resolved and the problem persists, the PCM might be at fault and requires professional evaluation.
What to fix
Based on the diagnostic results, apply the appropriate remedy. The most common fixes are to replace the sensor or repair wiring or connectors. If the cooling system has issues like a stuck thermostat, fix that as well. Then reset or reprogram the code and verify with a test drive.
- Replace the coolant temperature sensor if resistance readings are out of spec or the sensor is physically damaged.
- Repair damaged wiring or connectors and ensure a solid electrical connection.
- Replace a faulty PCM only after confirming it’s not the sensor or wiring; PCM replacement should be a last resort.
- Correct thermostat or cooling system issues (e.g., stuck open) that cause abnormal engine temps and misreadings.
After performing fixes, clear the codes and perform a test drive to ensure P0117 does not return and the engine operates normally across temperature ranges.
Prevention and tips
Preventive maintenance can help avoid P0117 in the future. Regular inspection of the cooling system, sensor wiring, and timely replacement of worn components can reduce risk. Ensuring proper coolant levels and using the correct coolant type for your vehicle also helps maintain accurate temperature sensing.
Note: If you are not comfortable performing electrical tests or sensor replacement, seek a qualified technician. Incorrect wiring or sensor installation can cause further damage or inaccurate readings.
Summary
P0117 is a common DTC indicating the Engine Coolant Temperature sensor input is reading too low. Start with a visual inspection of the sensor and wiring, test the sensor and power/ground circuits, and replace the sensor or repair wiring as needed. Address cooling system issues if indicated, reset the code, and confirm through road testing. If the problem persists after these steps, consult a professional for ECU testing or replacement.
In the future, keeping an eye on coolant levels, ensuring the thermostat functions correctly, and verifying sensor connections during routine maintenance can help prevent P0117 from returning.