Faulty engine coolant temperature sensor; Damaged, corroded, or disconnected electrical connector at the sensor; Open or shorted wire in the sensor signal or ground circuit; Coolant leak or low coolant level causing the sensor to read air temperature; Failed thermostat stuck open, preventing the engine from reaching normal operating temperature; Corroded or damaged pins at the ECM connector; Aftermarket sensor with incorrect resistance characteristics

DTC codes / P01xx · Fuel and air metering / P0115

P0115Engine Coolant Temperature Circuit Malfunction

Q: What does code P0115 mean?

Engine Coolant Temperature Circuit Malfunction. P0115 means the engine control module (ECM) has detected a fault in the engine coolant temperature (ECT) sensor circuit, typically an open, short, or out-of-range signal. The ECM expects a voltage from the ECT sensor that corresponds to a plausible coolant temperature, and when that voltage stays fixed, jumps erratically, or falls outside the expected range for a set time, it stores this code.

P0115 means the engine control module (ECM) has detected a fault in the engine coolant temperature (ECT) sensor circuit, typically an open, short, or out-of-range signal. The ECM expects a voltage from the ECT sensor that corresponds to a plausible coolant temperature, and when that voltage stays fixed, jumps erratically, or falls outside the expected range for a set time, it stores this code.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaFuel and air metering

Symptoms you may notice

Illumination of the check engine light, often with no other noticeable symptoms
Poor fuel economy or rich-running condition because the ECM defaults to a fixed coolant temperature
Hard starting or rough idle when cold, especially in low ambient temperatures
Inaccurate or non-functioning temperature gauge on the dashboard
Cooling fan running constantly or not engaging at all, depending on the default strategy
Possible black smoke or strong fuel smell from the exhaust due to over-fueling

Common causes, most likely first

Faulty engine coolant temperature sensor
Damaged, corroded, or disconnected electrical connector at the sensor
Open or shorted wire in the sensor signal or ground circuit
Coolant leak or low coolant level causing the sensor to read air temperature
Failed thermostat stuck open, preventing the engine from reaching normal operating temperature
Corroded or damaged pins at the ECM connector
Aftermarket sensor with incorrect resistance characteristics

How to diagnose and fix it

Check coolant level and condition; top up or repair leaks as needed
Inspect the ECT sensor connector and wiring for corrosion, damage, or looseness
Measure the sensor resistance with a multimeter and compare to the manufacturer's temperature-resistance chart
Backprobe the signal wire at the ECM and compare voltage to the sensor output to rule out wiring faults
Test the thermostat operation by monitoring coolant temperature during warm-up
Replace the ECT sensor if resistance is out of specification or the signal is erratic
Clear the code and verify the repair with a scan tool live data stream during a test drive

What the ECT circuit hides from you

A stuck-open thermostat can set P0115 because the coolant never reaches the temperature the ECM expects within a calibrated time, even though the sensor itself is fine.
Low coolant level is a frequent culprit: the sensor sits in air instead of liquid and reports a temperature that does not match reality, triggering the rationality check.
The ECM often substitutes a fixed default coolant temperature (typically around 80–90°C) when the circuit fails, so the engine may run acceptably and mask the fault until fuel economy or cold-start behavior worsens.
Corrosion inside the sensor connector can create resistance that shifts the voltage signal without any visible damage, so always wiggle-test the harness while watching live data.
Replacing the sensor without checking the thermostat and coolant level is a common misdiagnosis that leads to a quick comeback with the same code.

Can I keep driving with P0115?

You can usually drive the vehicle short distances to a repair shop, but avoid heavy loads, long trips, and cold-start driving because the ECM may over-fuel the engine and the cooling fan strategy may be compromised. If the temperature gauge reads abnormally high or you notice coolant loss, stop driving and address the fault immediately.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.