This week my car's check engine light came on. I took it to Autozone and they gave me a print out of possible problems. I then took it to a local shop where a friend works and they checked it out as well. They told me my cat. needed to be changed and it'd cost me 900.00 because it's a direct fit cat. So... being a single mother strapped for cash I immediately searched for a better deal. I ended up buying a cat on Amazon prime for around 300.00 and paid that same shop 150.00 for labor. I drove that car home ( 20 min drive on expressway) and then to my boyfriends house (1 hour there, 1 hour back ) on the way home, my light comes back on. I also noticed my MPG was awful compared to before. The next morning I take my car back and explain to them what happened. The shop manager asked me if it was raining when it had came back on. ( not sure why that matters ) but yes it was raining. They claimed to have checked my wires, saying they were bad. I then drove to the nearest parts store and bought new lifetime wires (which I had purchased 7 months prior but the assoicate failed to take my info for that warranty) I replaced my wires and checked my spark plugs to make sure they were in place. The light went off. I drove around for about an hour to make sure the light didnt return. After work that afternoon I drove home again and then took a drive to an event (about a 30 min drive there and 30 back) Still no light. Now, I notice my MPG is great. This morning, raining again, I crank my car to drive to work and the light is back on. I contact my friend who works at the shop, he claims the cat. I bought is bad because I bought it online. (??dont' they have to pass a federal test in order to be boxed and sold and it's name brand, brand new) Where should I look next ? My oil was recently changed. I did have a scary incident occur with the gas hand about a month ago. I got home from work and noticed I had a half of tank in gas then minutes later on the way to the store I had a full tank. On the home from the store it went back to half a tank. Since then I have driven the car over 1000 miles with no issues. (mentioning that because maybe its relevant) Spark plugs are newish, replaced in DEC 2012.

I wouldnt be so worried but I need to pass emissions and renew my tags... yesterday

any ideas on a solution!?