What it means
A general fault has been detected in the engine coolant temperature (ECT) sensor circuit. The ECU uses ECT for fuel enrichment during cold start, fan control, and closed-loop entry timing.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The engine coolant temperature sensor reports coolant temperature to the control module, which uses it for cold-start fueling, cooling-fan operation, and warm-up decisions. P0115 sets when the module detects a general electrical malfunction in the ECT sensor circuit, rather than a specifically low or high signal.
Common causes
Most likely first.
- Coolant temperature sensor failed
- Wiring or connector fault
Symptoms
- Check engine light on
- Cooling fan runs constantly or not at all
- Cabin heater weak or excessively hot
- Possible long warm-up time or hard cold start
How to diagnose it
- Check freeze-frame data and compare ECT live data with ambient temperature before starting a cold engine, then watch whether it rises smoothly as the engine warms.
- Inspect the ECT sensor connector and nearby wiring for looseness, corrosion, coolant contamination, or damage.
- Check the coolant level when the engine is cold and look for leaks that could leave the sensor exposed to air.
- Use a wiring diagram to identify the ECT circuit and check its supply, ground, and signal for opens or shorts against the manufacturer's specification.
- If the circuit checks pass, compare the sensor's resistance or reported temperature with the manufacturer's specification as it warms.
Common fixes
- Reconnect a loose ECT sensor connector and secure the harness.
- Repair damaged wiring or replace a corroded connector in the ECT circuit.
- Correct low coolant or repair a leak, then refill and bleed the system as specified.
- Replace the ECT sensor if its output fails the manufacturer's specification.
Can I keep driving?
Check the temperature gauge and watch for overheating; stop the engine if it overheats or coolant is leaking. A faulty ECT signal can disrupt fan control and fueling, so avoid a long drive until the fault is diagnosed.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.