Summary
P0117 means the ECT sensor voltage signal is too low, indicating a coolant temperature below -40°F — a reading that's physically impossible under normal conditions. The PCM recognizes this as a circuit fault rather than an actual temperature reading. The most common cause is a wiring short to ground (40%), followed by a failed ECT sensor (35%). This is typically a wiring or sensor issue, not a mechanical cooling system problem, and the fix is usually straightforward once you trace the circuit.
Severity: Moderate
Safe to drive: Yes, with caution — the PCM uses a default temperature value
Repair cost: $5–$250 depending on cause
DIY difficulty: Easy to Moderate
What does P0117 mean?
The ECT sensor is a negative temperature coefficient (NTC) thermistor: as temperature goes up, resistance goes down, and the voltage signal to the PCM drops. At very low temperatures, resistance is very high and voltage is high. At very high temperatures, resistance is low and voltage is low.
P0117 triggers when the ECT signal voltage drops below the PCM's minimum threshold — typically around 0.05–0.10V. This voltage corresponds to a temperature reading of roughly -40°F (-40°C), which tells the PCM something is wrong with the circuit rather than the coolant being impossibly cold.
The most common electrical cause: the signal wire is shorted to ground somewhere between the sensor and the PCM, pulling the voltage to near zero. A sensor that has failed with an internal short circuit produces the same result.
When P0117 is set, the PCM substitutes a default temperature value (usually around 170°F) for all its calculations. The engine will run, but fuel mixture, timing, and fan control won't be optimized.