This is my first post so first off hello to everyone. I purchased a 91 GMC K3500 with a 5.7 I purchased it with the check engine light on. The seller said it was 02 sensor. It needed alot of work so I brought it home and fixed it put a TON of new parts including a brand new 02 sensor. Well now that I got it going again the check engine light still goes on and off. It will be off when I first start the truck but after 5-10 minutes of driving it will come on. If I go into a store for a while and come out it will be off again until I get going again I think it turned off by itself while driving but not positive on that since I have only driven it once. THe truck sat in my garage for a couple of months with no battery so I know the code should be cleared. I stopped at Murry's and the code scan said it was the O2 sensor but its brand new so what else could be causing these troubles without it actually being the O2 sensor. I only saw one sensor before the cat and and the O2 sensor has a single wire. I hope I have provided enough info and thank you in advance.