Hi all. I have a 2002 A4 3.0 Quattro that has been misfiring. The OBD-II reader threw a P0300 code, and all of the cylinders had misfired except cylinder 2. I tested the ignition coils, which were fine, and so I then replaced the spark plugs. After resetting the stored codes, I took it for a short drive. It appeared to be misfiring for the whole time, and after checking the OBD-II codes, it appeared that only cylinder 5 had been misfiring.

I suspect that these issues may be caused by an electrical system issue. The car thinks the parking brake is always applied, I get a red ABS light all of the time, and now this misfiring. Is it likely a problem with some computer or bus? How should I move forward and troubleshoot the issue?

 

The brake light switch can cause misfires, or so I have read.

Are you ONLY getting the p0300 code? If you are using a non-vag scanner (generic one), it won't read any of the other modules in the car. It will only read the ECU and the TCM modules...all the others can generally only be read with a VAG Scanner or a high end scanner. So, you may have more codes that you are not seeing.

Your ABS and brake light could be on for various reasons, it can be the sensor on the wheel, brake pad wear indicator, wiring etc...generally if your ABS light isn't flashing at you, it's something minor...but again, the brake switch can cause misfires, loss of power etc..