P0115 P0xxx · Powertrain (P) Medium severity
Engine Coolant Temperature Circuit Malfunction
The ECT signal is out of range or implausible. The ECU then cannot judge warm-up state, so fuelling, fan control and transmission behaviour all go wrong.
Severity guidance: Medium — drivability or emissions affected, book it in this week
Symptoms you will notice
- Hard cold start
- Cooling fans running constantly or never
- Rich running
- Temperature gauge dead or pegged
Common causes
Ordered from most to least likely — start at the top.
- 1 Failed coolant temperature sensor
- 2 Open or shorted wiring
- 3 Low coolant level causing the sensor to read air
- 4 Corroded connector
How to diagnose it
Cheapest and least invasive first — most of these codes are not a parts problem.
- ✓ Compare ECT with intake air temperature after a cold soak — they should be within a few degrees.
- ✓ Check the coolant level before blaming the sensor.
- ✓ Measure sensor resistance across the pins and compare with the temperature/resistance table.
- ✓ Inspect the connector for coolant ingress (a known issue on several brands).
Repair options
- Replace the coolant temperature sensor
- Repair the wiring
- Top up and pressure-test the cooling system
P0115 — frequently asked questions
Why does the fan run with a cold engine? +
When the ECU loses the ECT signal it drives the fan as a protective default.
Can low coolant trigger this code? +
Yes — a sensor sitting in an air pocket reads nonsense.
Confirm the diagnosis before you buy parts
Send us the code, freeze-frame data and the vehicle — our engineers will tell you what to test first.