I had a Greddy cat-back on my 99 Civic and I traded it for a stock system and some cash but when I put the stock system on I got a CEL. The rear o2 sensor wasn't plugged in when the Greddy was on because it didn't have a cat, it had a test pipe. Now that the rear o2 sensor is in, would that make it throw a CEL? I'm just confused.

 

Pull the code. No one can help you without at least that information.

 

Got the code and it was the rear o2 sensor. It wasn't plugged in when the Greddy was on cause it didn't have a cat. Now it is plugged in with the stock system and threw a cel. Well I guess that's figured out.