What does it mean?
P1256 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Engine Coolant Temperature (ECT) Sensor, Open circuit/short to B+ .
Symptoms
- The MIL is on and the temperature gauge stays at the cold end or reads erratically
- Cooling fans may run continuously or not at all
- Very rich running, black smoke and a strong fuel smell
- Hard hot starting and rough idle after a short stop
- Poor fuel economy and sluggish throttle response
- Heater delivers hot air while the gauge insists the engine is cold
Likely causes
- Broken or corroded signal or ground wire between sensor and module
- Connector not fully seated, or a terminal pushed back in the housing
- Open circuit inside the coolant temperature sensor element
- Signal wire chafed against another circuit and picking up battery voltage
- Green corrosion in the connector creating high resistance after coolant seepage
- Damaged loom after cooling system or timing belt work in the same area
- Failed module input once the harness has been checked
How to fix it
- Look at the coolant temperature value on the scan tool; a fixed extreme cold reading on a warm engine indicates an open or high voltage circuit
- With the ignition on and the sensor unplugged, bridge the signal and ground pins in the connector; the reading should jump to maximum temperature, which proves both wires are intact
- If the reading does not change, test the signal and ground wires for continuity back to the module
- Check for battery voltage on the signal wire with the sensor unplugged to expose a short to power
- Inspect the connector for corrosion, coolant residue and spread terminals, and clean or renew it
- Measure the sensor resistance and compare with the specification for its actual temperature
- Replace the sensor or repair the wiring as proven, then watch the temperature climb steadily during a warm up cycle