EML & misfire

Q My grandson’s 2005 Vauxhall Corsa 1.2 Twin Port starts OK, but doesn’t pull as well as it did – after a few hundred yards, the control indicator for exhaust comes on. The handbook says this indicates emission values may be exceeded and my Maxiscan 509 shows fault code ‘P0304’ and a misfire on cylinder 4, but I have not had time to remove the coil pack and look at the spark plug. Would the misfire cause the control indicator light to come on? I had a problem like this myself some years ago on a Mazda and it was the coil pack to blame. Albert Kirby A Yes, the misfire and the fault code would cause the light to illuminate and any misfire will affect the emissions of the vehicle. If the cylinder is not firing, the unburned gases will escape through the exhaust, pushing up the HC levels. The code ‘P0304’ indicates a misfire on cylinder 4 and, as you suspect, the most likely cause is either a faulty spark plug or coil pack/ht lead.

Another possibility is that an injector has failed or you have a burnt valve. In the first instance, I would remove and check the spark plugs and, if nothing obvious comes to light, I would perform a compression test while the plugs are out to ensure that a deeper mechanical problem isn’t causing the misfire.

If nothing untoward has been discovered, check the connecting plugs to the injectors. Hopefully, by this stage you will have discovered the source of the misfire. If not, I would suggest swapping number four injector for one of the other units to check if the misfire switches cylinders with the injector. This would then indicate a faulty injector.