P0117
Engine Coolant Temperature Circuit Low Input
Powertrain Engine Cooling Coolant Temperature Sensing 🟡 Moderate — Fix within a week
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What This Actually Means
In plain language — no jargon

Your engine's coolant temperature sensor is sending a signal that's too low, like a thermometer stuck showing freezing when it's actually warm. The ECU thinks the engine is colder than it really is, which causes it to adjust fuel and ignition timing incorrectly.

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Symptoms You May Notice
3 known symptoms for this code
Engine runs rich with excessive fuel consumption
Difficulty starting or rough idle when cold
Check Engine Light illuminated
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Embedded Systems Insight
What the ECU/ECM is actually computing

The ECM monitors voltage from the coolant temperature sensor (CTS), which is a thermistor that changes resistance based on coolant temp. When coolant heats up, resistance decreases and voltage drops. P0117 triggers when the sensor voltage is abnormally high for too long, indicating a temperature reading below the minimum calibrated threshold.

Voltage & Parameter Thresholds

Parameter Normal Range Fault Condition
CTS Voltage 0.5-4.5V (corresponding to -40°C to 125°C) >4.5V or equivalent to <-40°C for >5 seconds
Sensor Resistance Decreases as temperature increases Open circuit or extremely high resistance detected
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Diagnostic & DIY Fix Guide
Check these in order — from cheapest to most complex
1
Wiring and connector
Inspect CTS connector for corrosion, loose pins, or water intrusion and clean or reseat as needed.
2
Engine Coolant Temperature Sensor
Replace the CTS after confirming wiring is good; it may have failed internally or developed an open circuit.
3
ECM connector and harness
Check the ECM-side connector and wiring for damage or corrosion if sensor and wiring test normal.