What does it mean?
P1240 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Cylinder 4 Fuel Injection Circuit Open Circuit .
Symptoms
- Cylinder 4 does not fire, giving a hard misfire and a flashing check engine lamp
- Heavy shaking at idle and very poor acceleration
- Unburnt mixture entering the exhaust, which can overheat and damage the catalyst
- Rough, uneven exhaust pulse and higher fuel use
- Engine may stall when coming to a stop
Likely causes
- Wire broken or pulled out of its terminal, often after previous work on the engine
- Injector connector loose, unlatched or with a corroded terminal
- Injector coil open internally
- Supply feed to the injector interrupted by a blown fuse or an open joint in the harness
- Damaged loom section from heat, chafe or rodent activity
- Open circuit in the module output stage
How to fix it
- Note whether the fault is limited to cylinder 4 or shared with other injectors
- Check the fuse and measure supply voltage at the injector connector with the ignition on
- Inspect the connector body for terminal damage, corrosion and a broken retaining tab
- Measure injector resistance and compare it against the other cylinders
- Test the control wire for continuity between the injector and the module while flexing the harness
- Confirm with a noid light whether the module is pulsing the injector
- Repair the open wiring or fit a new injector as the results indicate, then recheck the misfire data
- Inspect the module driver only after the external circuit is proven