What does it mean?
P1243 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Fuel Injector Circuit/Open Cylinder 4 . This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms
- Cylinder 4 contributes nothing, producing a hard misfire and a flashing check engine lamp
- Very rough idle and severe vibration through the vehicle
- Marked power loss, especially under acceleration
- Fuel and air passing unburnt into the exhaust, which can overheat the catalyst
- Fuel consumption rises and the engine may stall at idle
Likely causes
- High resistance or a complete break in the injector control wire, often at a flex or repair point
- Terminal fretting inside the connector, where vibration has worn the contact until it opens
- Injector coil failed open through age, heat or a previous electrical fault
- Connector left partly seated after work on the intake or the fuel rail
- Loss of the common supply to the injectors from a blown fuse or an open splice
- Open circuit or a cracked solder joint in the module output stage
How to fix it
- Read all injector codes together; a group of them normally means the shared feed is at fault
- Check the injector fuse and measure supply voltage at the cylinder 4 connector with the ignition on
- Unplug the connector and inspect the terminals for fretting, corrosion and loss of tension
- Measure the injector winding; an infinite reading confirms an open injector
- Check continuity of the control wire back to the module and move the harness while measuring to catch an intermittent open
- Use a noid light to see whether a drive pulse is present at the connector
- Repair the wiring or fit a new injector as the tests dictate, then confirm the cylinder now fires using live misfire data
- Test the module driver only after the harness and injector are proven good