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depends on how the motor itself is running. if it has low miles, doesnt burn oil or blow smoke you should be ok. but if your going to replace the bottom one why not just get a hi flow cat to replace both. it is legal you know.
If you are worried about emmissions and don't want to spend money with CATs, I'd recommend getting SEA FOAM to clean out the insides of your engine. Follow directions printed on can. It will be the best $5.00 you will spend...
Just replace the stock Y-pipe with the Bassani unit that has the high-flow CAT in it already. Gets you everything back to the muffler new, stainless, and will bolt right up to the stock manifolds without modifications. Of course there is the dilema of having 3" diameter exhaust back that far and a stock muffler....hmmmm. OK, well it was a nice idea while it lasted.
Just keep in mind that even if you don't have inspections in your area, the deliberate removal of emissions equipment isn't a great idea.
Oh yeah, we have emission inspections and it is a royal pain if you fail them. I was just trying to get out of replacing both cats. It's tight out here since our air quality is getting bad ie. Phoenix is becoming "Little LA".