What does it mean?
P1263 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Cylinder 3 High To Low Side Short .
Symptoms
- The MIL is on and often flashing while the misfire is active
- Steady misfire from cylinder 3 with a rhythmic shake at idle
- Loss of power and rough acceleration
- Smell of unburnt fuel and higher fuel consumption
- Rattling or popping from the exhaust on some engines
- The engine may run better once warm but the fault returns under load
Likely causes
- Insulation worn through between the two wires of the cylinder 3 injector circuit
- Injector connector for cylinder 3 full of oil, coolant or water, bridging the pins
- Internally shorted injector winding on cylinder 3
- Harness pinched under the intake manifold or a rocker cover during previous work
- Damaged terminals in the cylinder 3 connector after repeated disconnection
- Aftermarket wiring or a poorly made splice in that injector circuit
- Module driver damaged by the short, confirmed after the external circuit is repaired
How to fix it
- Confirm on live data that the misfire counter is rising on cylinder 3 only
- Unplug the cylinder 3 injector and inspect the connector for oil, water and green terminals
- Measure the injector winding resistance and compare it with the other injectors on the engine
- Check insulation between the two injector wires and between each wire and ground with the connector off at both ends
- Swap the suspect injector with one from another cylinder and see whether the fault follows it
- Inspect the loom where it passes under the manifold for pinch and chafe damage
- Repair the shorted wiring or fit a new injector for cylinder 3, then clear the code and verify the misfire counter stays at zero