P0117
UniversalEngine Coolant Temperature Circuit Low Input
Medium SeverityThe Engine Control Module (ECM) detects a voltage reading from the Engine Coolant Temperature (ECT) sensor that is lower than expected. This usually indicates the sensor is reporting an extremely cold engine temperature, causing the ECM to adjust fuel delivery incorrectly and affect engine performance and fuel efficiency.
⚠️ Before You Begin (Safety First)
Always allow the engine to cool completely before working on the cooling system to avoid severe burns from hot coolant or engine parts. Disconnect the battery before performing any electrical repairs to prevent accidental shorts or shocks. Use appropriate eye protection.
Common Symptoms
- Check Engine Light illuminated
- Poor fuel economy
- Engine runs rich (excess fuel)
- Hard starting, especially in cold weather
- Black smoke from exhaust
- Engine may run rough or idle poorly
- Overheating (in some paradoxical cases where ECM gets bad data)
Common Causes
- Faulty Engine Coolant Temperature (ECT) sensor
- Short circuit to ground in the ECT sensor signal wire
- Corroded or damaged ECT sensor connector
- Open circuit in the ECT sensor wiring
- Faulty Engine Control Module (ECM) - rare
Frequently reported on: Honda Civic (2001-2005), Ford F-150 (2004-2008), Toyota Camry (2002-2006), Chevrolet Silverado (2000-2006)
Diagnostic Steps
- Verify the code with an OBD-II scanner and clear it to see if it immediately returns.
- Locate the Engine Coolant Temperature (ECT) sensor, usually on the engine block or cylinder head near the thermostat housing.
- Visually inspect the sensor and its electrical connector for any obvious damage, corrosion, or loose connections.
- Disconnect the electrical connector from the ECT sensor.
- Carefully remove the old ECT sensor (coolant may leak, so have a drain pan ready).
- Install the new ECT sensor, ensuring it is tightened correctly but not overtightened.
- Reconnect the electrical connector firmly.
- Top off any lost coolant, bleeding air from the system if necessary.
- Clear the P0117 code using your OBD-II scanner and test drive the vehicle to confirm the repair.
👨🔧 When to Call a Professional
While replacing the ECT sensor is often a straightforward DIY task, if the code persists after replacement, it suggests a more complex electrical issue, such as a wiring harness problem or even an ECM fault. In such cases, it is best to consult a professional mechanic for further diagnosis, as specialized tools are often needed to trace electrical shorts or open circuits accurately.
Commonly Asked Questions
What does 'low input' mean for the ECT sensor?
A 'low input' means the sensor is sending a voltage signal that is too low. For an ECT sensor, a low voltage usually corresponds to a very cold temperature reading. The ECM interprets this as the engine being extremely cold.
Can a bad ECT sensor cause my car to fail emissions?
Yes, a faulty ECT sensor reporting incorrect temperatures can cause the engine to run rich, leading to excessive emissions of hydrocarbons and carbon monoxide, which will likely result in an emissions test failure.
Is it safe to drive with a P0117 code?
While the car may still be drivable, it's not recommended for extended periods. The incorrect temperature readings can lead to poor fuel economy, rough engine operation, and potentially damage the catalytic converter over time due to rich fuel conditions.
How can I test the ECT sensor myself?
You can test an ECT sensor by checking its resistance with a multimeter. As the engine warms up, the resistance should decrease. Consult your vehicle's service manual for the correct resistance values at specific temperatures.
Do I need to drain all the coolant to replace the ECT sensor?
Not necessarily all of it. You may only need to drain a small amount of coolant, usually enough to get the coolant level below the sensor's mounting point. Have a catch pan ready to minimize mess.