Code P0115: coolant temperature sensor: circuit fault

Original definition (SAE J2012): Engine Coolant Temperature Circuit Malfunction^[1][2]

• P0115Engine coolant temperature circuit (ECT sensor): malfunction.
• P0116Engine coolant temperature circuit: range or performance problem. The sensor signal falls outside the expected range or is not consistent with the engine's actual behaviour.^{[7][12][13][14][15]}
• P0117Engine coolant temperature circuit: low input signal, meaning the sensor voltage is below the expected range, normally because of a short to ground in the sensor or its wire.^{[8][9][16][17][18]}

1. 1Faulty coolant temperature sensor: this is the first cause cited by Autodoc and CarParts.^{[1][3][5][6][10]}
2. 2Sensor wiring and connectors in poor condition: damaged, chafed or corroded wires, loose or dirty connectors, water in the connector, a short circuit in the signal wire or an interrupted earth circuit.^{[1][3][6][10]}
3. 3Cooling system problems that distort the reading: low coolant level, air in the system, a faulty thermostat or engine overheating.^[1][3][6]
4. 4Faulty ECU (ECU/PCM): this is the least likely cause.^[1][3]

• Engine warning light on in the instrument cluster.^[3][6][10]Engine warning light: faults that cause it
• Difficulty starting, especially with a cold engine.^[1][3][5]Hard to start: faults that cause it
• Rough idle, or idle at a higher speed than normal.^[1][5][14]Vibrations: faults that cause it
• Increased fuel consumption.^[1][3][4][5]Uses more fuel: faults that cause it
• Black smoke from the exhaust, because the ECU enriches the mixture too much.^[4][16]Black smoke: faults that cause it
• Reduced engine performance, especially when cold.^[1][16]Less power: faults that cause it
• The engine may stall.^[3][9]Engine stalls: faults that cause it
• Engine overheating if the radiator fan switches on late or never switches on.^[3][4][16]Overheating: faults that cause it
• Radiator fans running non-stop or behaving abnormally.^[1][6][10]
• Temperature gauge giving wrong readings.^[1][6][10]
• The air conditioning may stop working.^[6][10]

1. 1Read the code and the live data with a diagnostic scan tool: the coolant temperature should rise consistently as the engine warms up. With the engine switched off for several hours, the coolant, oil and intake air temperatures should match to within about 5 °C at most; if one sensor deviates by more than 10 °C, it is the suspect. A disconnected sensor usually shows up on the scan tool as −40 °C.^[6][10][18]
2. 2Check the coolant level with the engine cold and make sure there is no air in the system. This is the cheapest check and can also distort the reading.^[1][3][6]
3. 3Inspect the sensor connector and wiring: corroded terminals, water in the connector, chafed wires. Wiggle the wiring loom at several points and in several directions while watching the reading to locate intermittent breaks or short circuits.^[6][10]
4. 4With a multimeter, measure the sensor's resistance (high when cold, low when hot) and compare it with the manufacturer's curve, check that about 5 V reaches the connector and verify the continuity of the wiring.^[5][6][18]
5. 5Only when the wiring and voltage are fine, replace the sensor. If the reading is still not consistent, check the thermostat and, as a last step, the ECU.^[1][3][5]

• Thermostat

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