General Maintenance & Repairs - 02 sensor question

K, i searched and was just not successful.

I failed emissions and have new cats. So my programmer hit on the code P0420.

I want to do this cheap as possible. So should I replace the upstream O2 sensors first or just replace em all?

2001 LS1 Trans Am. SLP headers, cats and loud mouth.

Not sure which ones you should replace but when you do, get the cheaper Denso's. They're the same exact thing you would get if you paid almost twice as much for AC Delco's.

If you're doing it yourself, just replaces the upstream o2 sensors to begin with. Also, don't know where you plan on ordering them from, but rockauto is where i plan to get mine when I LTs. I'd suggest the denso o2 sensors as well.

I ordered up the denso's off ebay, just bought all 4 of them. cut the price in half :)