Will a bad o2 sensor ruin a new cat?

[Raven Tooth]

Just moved to the great state of California from Florida. Need to get my car to pass smog. Just went to get a test and I failed miserable, I am a "gross polluter." The smog tech said it's likely my cat is useless, especially since I failed by so much, so I'm looking into getting a new one (probably something cheap that will at least let me pass once and give me two more years...).

Since this is all unplanned spending so I'm trying to get by as cheap as possible. I've heard from a friend with an e36 that if my 02 sensor is bad and I replace the cat without replacing it, then it can ruin my cat in like a month. Is this true? Are e36s different in this regard? My car is running pretty good so I never thought the o2 sensor could be bad or that bad, but it like is pretty old. Just want to know if I can get away without it or if I'm screwing myself in the end.

[nando]

if your engine is running really rich, yes it will ruin the cat. a stock cat is definitely far better than an aftermarket cat, but if you can't afford one ($$$) then you're stuck with aftermarket.

[NKRoberts]

Yes if the car isnt getting the right information (bad o2 sensor) then it will ruin the new cat fairly quickly. I hate unexpected expenses like this as well but if you get a new cat get a new o2 sensor, you're car will thank you.

[Raven Tooth]

Thanks. I got a new cat and a new o2 sensor and my car still failed by the same amount! Need to start a new post... Somehow my car is actually running way too lean.

[nando]

vac leak after the AFM will make it run lean

did you reset the ECU after replacing the O2 sensor? Disconnect the battery for 5 minutes, then drive the car for a good 100 miles before you test again.

[matt]

Post your emissions sheet, we'll help you figure out what's wrong.