Cel came on last week for po420 catalyst system effciency below threhold bank 1 I went to advance and had the light turn off and put a gas treament a week later it came back on....any ideas.......please help.....

 

Bad gas can cause that code. Assuming you are sure that that is the only code, then I'd reset the code and run a whole tank of good gas through the system first.

According to the service manual, if that is the only code and it keeps coming back, it's a bad CAT on that bank (rear CAT).

Likely, it's either a spurious signal (they do happen), it was caused by bad gas, or you have an associated code along with it (bad A/F or O2 sensor)....

 

It when thru a few tanks of gas since the first time it happen. And I think the dealer change the o2 sensor about a year ago for a different problem it had....