One/some/all of my spark plug might be fouled up and needs to replace, if so, how many do I need to replace for a V6 engine car? Just one or six spark plugs?

 

Your best bet would be to check the engine codes. That should tell you exactly which ones are bad. I recommend changing them all. If one is toast, the rest may be following suit.

 

What is the cat pipe size for this car?

2002, Audi A4, Quarttro 3.0, V6

2.25", 2.5", or 3.0", they have this three to choose fromt he ECS tunning universal cat page, which one should I buy for the car above to replace?

Thanks

 

why do you want to replace your cat? if you have under 80k its still under waranty.

 

Yes deff check for codes, dont do anything until you do. Then you will know what to do. any questions you know where to go