This forum has helped me out alot in solving problem with my 1997 KIA sephia GS 1.8l.. Recently my check engine light came on and went to local shop to get scanned and the code was P0420 catalyst efficiency in bank #1.. WHat does it mean? how can i fix?? I recently changed both 02 sensors... Could they have gone bad so soon.. My car some times shuts off while driving,, and the rpm gauge idles funny, the car shuts off but turns back on again... Hopefully we can resolve this problem??? Thank You once again.. oh my car gas 37k miles....