I have a A4 2005 1.8T with 55000 miles. The problem started about 7 months back. CEL light on (steady not flashing). Autozone check showed Cylinder 4 misfire (P0304). Reset the code first time to check if it comes back, and yes sure it did. Dealership recommended updating the ECU software. Did that but code still came back same. Next got the all 4 ignition coils replaced. still the code comes back as cylinder 4 misfire. swapped the spark plugs from cylinder 4 to cylinder 3, and the code still comes back as cylinder 4 misfire. I am just not sure what to do next.

Only thing noticeable is the gas mileage has gone down 4-5 miles per gallon. any ideas or suggestions what else to look for. Thank you in advance

 

Did you swap the plugs and the coils? or just the plugs. If you did just the plugs, swap the coils around. If the misfire follows the coil, it is bad. If the misfire stays in cylinder 4, there may be something else wrong like fuel injector, valves, wiring to the coil...

 

yes the ignition coils (cylinder 4 to cylinder 2) and the spark plug (cylinder 4 to cylinder 3) were swapped but the misfire still remains in cylinder 4.

 

That means you have either a fuel or mechanical problem. Seeing as how rare mechanical problems are, I would go with a fuel problem. Check out your fuel injectors. I wouldn't waste time or money with the filter, that would affect them all. You will be looking for something that only affects number 4. Check to make sure it is plugged in. If you have access to a noid light, use it. If you are lucky the computer will log an electrical problem with the injector.