Engine Coolant Temperature Circuit Low Input
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.
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.
| 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 |
Code P0117 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0117 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.