ok i have a 97 gst stock i had just replace my upstream o2 sensor because it was completely dead and not working anymore it even threw a cel code and before the o2 died it use to run rich once i gas it and let off the throttle it'll stay full rich and evens out again to 14.8-15.1 and now that ive replaced it with the new o2 sensor when i gas it it goes rich and when i let off the throttle it stays super rich than it moves its way to super lean and stays there until i gas it again and it does the same thing over unless i let it sit at idle for acouple of minutes than it goes back to normal reading 14.8-15.1