Engine Coolant Temperature Circuit Low Input
Diagnostic trouble code P0117 indicates that the Powertrain Control Module (PCM) has detected an excessively low voltage signal from the Engine Coolant Temperature (ECT) sensor circuit. Because ECT sensors are typically negative temperature coefficient thermistors, a low voltage input translates to an abnormally high engine temperature reading.
This code requires immediate attention because the engine computer relies on accurate temperature data to manage fuel delivery and cooling fans. Driving with this code can lead to poor drivability, stalling, or severe engine damage if the vehicle is actually overheating.
Common symptoms
- Illuminated Check Engine Light
- Engine cooling fans running constantly
- Hard starting or failure to start when the engine is cold
- Rough idling and poor engine performance
- Decreased fuel economy due to an improper fuel mixture
Likely causes
- Faulty Engine Coolant Temperature (ECT) sensor
- Short to ground in the ECT sensor wiring harness
- Corroded or damaged ECT sensor electrical connector
- Engine overheating due to low coolant or mechanical failure
- Failed Powertrain Control Module (PCM) (rare)
What to check before replacing parts
Check the engine coolant level and condition.
Look for: Ensure the radiator and reservoir are full of clean coolant, as low coolant can cause actual overheating that triggers the code.
Visually inspect the ECT sensor, wiring, and connector.
Look for: Look for melted wires, rubbed through insulation causing a short to ground, or severe corrosion inside the connector.
Monitor the ECT sensor data using an OBD-II scan tool.
Look for: Check if the live data shows an impossibly high temperature (e.g., 250°F+ or 120°C+) even when the engine is cold, which confirms a sensor or circuit short.
Disconnect the ECT sensor and recheck the scan tool data.
Look for: If the temperature reading drops to an extremely cold value (e.g., -40°F) when unplugged, the sensor itself is internally shorted and needs replacement.
Test the ECT sensor wiring with a digital multimeter.
Look for: With the sensor unplugged, check for a 5-volt reference signal and a good ground; if the 5V wire has continuity to ground, the wiring harness is shorted.
This page explains the generic P0117 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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