Summary
P0118 means the ECT sensor voltage signal is too high, indicating a coolant temperature of approximately -40°F or lower — a physically impossible reading under normal conditions. This signals an open circuit in the ECT wiring or a sensor that has failed open. The most common cause is an open circuit in the ECT wiring (40%), followed by a failed ECT sensor (30%) and a disconnected connector (20%). This is an electrical fault, not a sign of actual engine overheating, and is usually a quick fix.
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 P0118 mean?
The ECT sensor is a negative temperature coefficient (NTC) thermistor: its resistance is very high at cold temperatures and drops as the coolant heats up. The PCM sends a 5V reference through a pull-up resistor to the sensor. When the sensor resistance is high (cold), the return voltage is high. When resistance is low (hot), the voltage is low.
P0118 triggers when the ECT signal voltage exceeds the PCM's maximum threshold — typically around 4.9–4.95V. This corresponds to an extremely high resistance (or an open circuit), which the PCM translates to a temperature reading of about -40°F. Since that's not physically happening, the PCM knows the circuit is broken.
The critical detail: P0118 (high input / high voltage) means the circuit is open, not shorted. The voltage goes high because the PCM's pull-up resistor has no path to ground through the sensor — the circuit is broken somewhere. This is the opposite of P0117, where the voltage goes low due to a short to ground.
When P0118 is active, the PCM substitutes a default temperature for all calculations. The engine will run but with compromised fuel control, fan operation, and shift timing.