I have an 06 Expy 5.4 3v with approx 70k. When i first got her she had 62k on her and ran fine...then i got a cel..p0420..had that for a month..no problems then it went away...about 2 months later she started bogging down a bit nothing really bad and the p0420 code came back. After that I had a p0307 for a couple of days but it went away. About a month ago i had a bad miss at low speeds, i checked it and had the misfire code and the 420 code...replace the COP the miss went away...still had the 420 code though. I havent looked at the cats yet...I didnt replace the plug on cylinder seven either. Funny thing happened though...a friend and i were messing round at work and i did a burnout...the cel went out and it ran great for a couple of days..but now she bogs down and the cel light flashes when im kicking it down from a start or to pass..the p0307 code came back and now i sometimes hear what sounds like a chugging or dieseling sound when i let off the accellerator pedal or climbing a hill. Should I replace the plug? Should i pull off the cats and replace them? Could it possibly be a fuel injector...any help you guys can give me will save me some money in the long run i hope...thanks...