I have a friend that has a 2003 Toyota Matrix and she has a check engine light,P0420,cat is not working properly. Ive already changed the spark plugs,and before i go tell her to take it to toyota dealer ship should i replace the Oxygen senors first?The one after cat??Or should I let toyota Diag it and charges me 50$ and tells me that it might me the O2 sensor?

Check this page out:
http://www.obd-codes.com/p0420