P0115: Engine Coolant Temperature (ECT) Circuit

P0115 means the ECU has detected a general fault in the engine coolant temperature (ECT) sensor circuit. The ECT sensor is a thermistor whose resistance changes with temperature, telling the ECU how warm the coolant is so it can adjust fuelling, ignition timing and the cooling fan. A fault here usually points to a failed sensor, damaged wiring or a bad connector. Repair typically costs 40 to 180 EUR depending on whether you DIY or use a shop.

P0115 is a cooling system-related diagnostic trouble code classified as moderate severity. When your vehicle's ECU detects this condition, it stores P0115 and illuminates the check engine light.

If your vehicle has triggered P0115, you may notice one or more of these symptoms:

• Check engine light illuminated
• Hard cold starts or rough running when cold
• Poor fuel economy from over-rich fuelling
• Temperature gauge reading incorrectly or stuck
• Cooling fan running constantly or not at all
• Black smoke from the exhaust on a cold start

Here are the most common causes of P0115, ranked by how likely they are to be the culprit:

Generally yes, for short-term driving. Usually safe to drive short distances. P0115 is a general fault in the coolant temperature sensor circuit, so the engine cannot read true coolant temperature reliably. The ECU defaults to a fixed temperature value, which richens cold-start fuelling and can force the cooling fan to run constantly, hurting fuel economy. Because the gauge may read wrong, you lose your early warning for real overheating, so watch for steam or rising temperature and address it promptly.

Follow these steps to pinpoint the root cause of P0115:

1. Read the code and watch the live coolant temperature PID with Skanyx. On a cold engine it should read close to ambient air temperature, then climb steadily to around 85 to 95 C as the engine warms
2. If the live value is stuck at a fixed default (often -40 C or 120 C/maximum), that points to an open or shorted circuit rather than a true temperature reading
3. Inspect the ECT sensor connector for corrosion, bent pins, or a loose fit, and check the wiring for chafing or rodent damage
4. Measure the sensor resistance with a multimeter and compare against the manufacturer resistance-versus-temperature table
5. Check coolant level, because a low level can leave the sensor tip in air and skew the reading
6. If the sensor and wiring test good but the fault persists, check the sensor ground and signal continuity back to the ECU

Repair costs for P0115 vary depending on the root cause and your vehicle. Here are typical estimates:

P0115 behaves differently depending on your vehicle. Select your car below for model-specific causes, known issues, and adjusted cost estimates:

These codes are often seen alongside P0115 or indicate related issues: