Summary
P0115 means there's a general fault in the engine coolant temperature (ECT) sensor circuit. The PCM is getting a signal from the ECT sensor that's out of its expected voltage range — either erratic, missing, or stuck. The most common cause is a failed ECT sensor (50%), followed by wiring or connector damage (30%). This is a moderate-severity code that's usually an inexpensive fix once you pin down whether the sensor or the wiring is at fault.
Severity: Moderate
Safe to drive: Yes, with caution — the PCM will use a default coolant temp value
Repair cost: $10–$250 depending on cause
DIY difficulty: Easy to Moderate
What does P0115 mean?
The engine coolant temperature sensor is a thermistor — a resistor that changes its resistance based on temperature. As the coolant heats up, the sensor's resistance drops, and the voltage signal it sends to the PCM changes accordingly. The PCM uses this data for fuel mixture, ignition timing, cooling fan activation, and transmission shift scheduling.
P0115 is a general circuit malfunction code. Unlike P0117 (low input) or P0118 (high input), which point to a specific voltage direction, P0115 indicates the PCM sees something broadly wrong with the circuit — the signal may be erratic, stuck at a fixed value, or jumping between extremes. The PCM can't trust the reading and falls back to a default temperature value, which affects fuel economy and driveability.
On most vehicles, the PCM sets P0115 when the ECT voltage is outside its valid range (typically 0.1V–4.9V) or when the signal doesn't change as expected over time.