Engine Coolant Temperature Sensor Circuit Low Input
ECT signal reading lower voltage than possible — usually shorted-to-ground wire or failed sensor.
What it means (plain English)
The ECT sensor is a thermistor — its resistance changes with temperature. The PCM feeds it 5 V through a pullup resistor and reads the voltage drop. Cold = high resistance = high voltage. Hot = low resistance = low voltage. P0117 sets when the voltage drops below the floor (~0.1 V), which means electrically the sensor is reading hotter than physically possible. Almost always a shorted wire or a failed sensor that's gone dead-short internally.
What the computer is actually seeing
ECT signal voltage below the calibrated minimum (typically <0.2 V), indicating an impossible high temperature.
What a healthy reading looks like
Cold engine (70°F): ~3.5–4 V signal. Operating temp (195°F): ~1.0–1.5 V. Should sweep smoothly as engine warms.
Guided diagnostic — the DiagCoach way
Don't just throw parts at it. Walk through these in order — each step tells you whether to keep going or stop and fix what you found.
- 1Unplug ECT sensor with key on. Voltage at signal pin should jump to ~5 V. If it stays low — wire is shorted to ground between sensor and PCM.
- 2If voltage goes high with sensor unplugged — sensor is bad, replace it.
- 3Inspect connector for bent / corroded pins.
Common causes
- Signal wire shorted to ground
- Failed ECT sensor (internally shorted)
- Corroded connector pushing pins together
Typical repair cost
$50–$200.
Related codes
Frequently asked questions
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