Hello, My 1991 Formula was garaged through the winter but now it is running kind of rough. I have added dry gas and fuel additive and it has a new tank of gas, but it is still running very rich (and failed emissions miserably), it hesitates sometimes when taking off, and between 30 and 40mph it feels like it's gagging slightly... don't know how else to describe it. I am told it might need a new O2 sensor. When I search for info about sensor brands, I see people talking about $300+ for O2 sensor, but when I search on Summit it's like 20 bucks?? Am I looking at the wrong thing? I am also going to replace spark plugs just in case because it's been a while.. planning to with iridium NGKs. Anyone see an issue with this? Thanks for any help

I had a similar problem with my car. It would hesitate on take off. I got a new o2 sensor from autozone for about 30 bucks (Bosch) and I also bought a new throttle position sensor for 50 bucks. This pretty much solved the problem. The o2 sensor was a bitch to change though. My suggestion is get an o2 socket wrench, about another 5 bucks and get some good oil (wd-40 or something better). Oil up the old sensor, run the engine and make it nice and hot on the exhaust and then very carefully change the o2 sensor (avoid burns). When I did mine the threads were shot and i had to take a drill and drill out part of the threads.

Quite a lot of us have had problems with Bosch O2 sensors failing quickly, and/or not reading properly, so the suggestion is to stick with genuine AC Delco O2 sensors. They should cost around $40-50 at your local auto parts store, or the dealer. An O2 sensor that costs $300 is probably an OBD2 model for '96+ vehicles.

That explains it then... I will get a new sensor and hope it passes emissions! Thank you both for your help