I recently changed my spark plugs on my 2003 audi a4 3.0L avant.
I did the work myself and took my time doing one at time.
I used the OEM plugs.
Everything was fine for over a month.
Then the dreaded Check engine light came on.
Code P0421
Says Warm Up Catalyst; Bank 1: Efficiency Below threshold

Do you think me changing the plugs caused this light?
Whats the normal fix?
Just need a new Cat?

ANY ADVICE MUCH APPRECIATED HERE!!!!
THANKS!

 

OO btw I used the fuel cleaner REDLINE!
I just realized that I used it on the tank right before the CHECK ENGNE LIGHT went on. Maybe the fuel cleaner caused an excess of fumes on this next tank??