OBD II Code question

I recently got a check engine light on my 97 GTP, and I got it checked at AutoZone. The code was a P0420, which was determined to be Catalyst System Efficiency Below Threshold (Bank 1). This code has come up before, and the last time it did, the guy there said that usually that is just an O2 sensor out of whack. Since the car was at about 100,000 miles I went ahead and changed both O2 sensors, changed plugs, and wires. The problem went away, and the car got the best mileage it has seen since I owned. It is now at 120,000 miles, and I am met with this code once again. I have heard about people just getting one of those O2 bypasses and just putting a straight pipe in there.
If anyone has any suggestions, that would be great.

The bank 1 sensor is the one located on the rear exhaust manifold and bank 2 is under the car after the cat.

They do not make a spoof for bank 1,it's only for bank 2 after the cat.

What kind of O2 did you replace it with ? I hope it wasnt a BOSCH.

~Larry~

It was indeed a Bosch....

What brand would you recommend?

I hit the nail right on the head :naughty: Ive seen alot of the Bosch not last very long....Only one is stock, AC-Delco ASF109 or GM part # 25312203 You can get one from www.gmpartsdirect.com for $73.70 shipped or the dealer list is $122.83

Off hand what kind of MPG were you getting before the O2 went bad ?

~Larry~

well, my best was 33mpg when I was going from Hutchinson, KS to Manhattan, KS.....very flat, very straight, I didn't go romping onto the freeway....I was seeing what I could get out of it.

Usually, though, I get about 26-28 on the freeway, and 19-24 in town.

Also, who does ACDelco get their O2 sensors from? If I am not mistaken, those are one of the parts that they don't produce themselves.

Also, who does ACDelco get their O2 sensors from? If I am not mistaken, those are one of the parts that they don't produce themselves.

Im not sure on that one.I thought that they made there own O2's.I know that on some of there spark plugs they are subing out to NGK.

Thanks for the info on the MPG !


~Larry~