2344018 is your front sensor, the one in your manifold. The one you need for after the cat I belive is this one STANDARD MOTOR PRODUCTS Part # SG277 It says downstream. So if that'* the wrong one, someone speak up. Why are you changing your rear sensor? Is something wrong with it? From what I understand you only need to change it if your getting codes.

About a month or 2 ago my SES came on, went to Autozone they scanned it and it said rear cat o2 sensor, so I bought a sesnor from them ( BOSCH ) didn't know that that wasn't a smart buy and the light is still on, cat is fine so its probably the sesnor still. SO I'm going buy a Denso sensor for the rear and now I'm thinking of doing the front as well because that has never been change and it has 120,000+ miles on it

OK, got it. As long as there'* a reason. Ya, the front one could use a change. Front will take you about 15 to 30 mins to change depending on if it comes out easy or hard.

Approx 30k for 1 wire unheated, ~100k for 3 or 4 wire heated. And it won't show symptoms for a good while..and by that time, you might have done some unnecessary damage to the emissions system, cat, etc.

On a system with an upstream and downstream, or dual, only replace the upstream, the downstream is there for the express purpose of throwing a code when the cat is no longer doing its job within spec, which usually means its clogging, and time for replacement.

Only use AC Delco or Denso sensors. ONLY.

These can be purchased at GMPD (confirm part number) or rockauto.com. eBay is also a good place to find one at a lower cost.

I also need to change my upstream o2 on my '00 SSEi because the car has 138k miles on it and had a previous blown intake (which may have caused the sensor to be coated in coolant. I was searching around and at O'Reilly Auto Parts, they offer a Delphi. Isn't Delphi in relation to AC Delco? Will a Delphi be acceptable?

Another thing, I swear I heard a guy I know, which is a technician at a GM dealer, has said that Bosch is making GM'* o2 sensors now. That is what I heard, I'm not sure if it is true or not.