What does it mean?
P1255 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Engine Coolant Temperature (ECT) Sensor, Short to Ground .
Symptoms
- The MIL is on and the temperature gauge reads much hotter than the engine actually is
- Cooling fans running constantly, even from a cold start
- Rich running, black smoke and a fuel smell from the exhaust
- Hard starting when cold because the module thinks the engine is already hot
- Noticeably higher fuel consumption and possible rough idle
- Heater blowing cool air while the gauge shows hot
Likely causes
- Signal wire chafed against the cylinder head, a bracket or the intake and shorted to ground
- Internal short inside the coolant temperature sensor itself
- Coolant or water leaking into the sensor connector and bridging the pins
- Crushed or melted section of loom lying against the exhaust manifold or a heater pipe
- Corroded terminals allowing moisture to track between signal and ground pins
- Incorrect or poor quality replacement sensor with the wrong resistance curve
- Failed module input stage after all wiring tests pass
How to fix it
- Compare the coolant temperature reading on the scan tool with an infrared or contact thermometer on the thermostat housing
- A reading pinned at the top of the scale with a cold engine points to a shorted circuit rather than an overheating engine
- Unplug the sensor with the ignition on; the reading should swing to the coldest extreme, which proves the wiring is not shorted
- If the reading stays hot with the sensor unplugged, check the signal wire for a short to ground along its route
- Clean and dry the connector, inspect for coolant seepage from the sensor body and check terminal condition
- Measure the sensor resistance cold and compare with the temperature to resistance table
- Replace the sensor with a correct specification part or repair the shorted wire, then confirm the temperature reading rises smoothly during warm up