New cat, Engine light, backfire

I put in a new Magnaflow cat. The cores in the old one were rattling around. It's quiet now. BUT, after about 10 minutes on the road, the engine light came on. When I rev the engine and back off, I get a little backfire sometimes. Should I replace the O2 sensor and how do I reset the Engine Light?

I would use a scan gauge and see what code was set(might not be the o2), reset it, and see if it comes back. If you disco the battery, the code will reset and your computer will as well and maybe when it relearns, it'll adjust for your now freer flowing cat.

I will definitely try that. I will do the battery disconnect. The problem started about 10 minutes after running with the new cat. I'll let you know what happens.

YES! Well, I disconnected the battery for about 30 plus seconds, reconnected, went for a 15 minute ride. The Engine light went off a few seconds after startup and did not come back on. Thank you much.
Walt

I would keep a close eye on that, maybe run some analog volt meter tests on the running O2 sensor. It may be running rich, and that may have killed the last cat. O2 sensors can go bad, and the heater wire voltage to the O2 sensor can go out with out throwing a code as I found out last week, causing a rich condition, backfires, and a destroyed cat in the process!
Computers can only tell you what they see! They are not omniscient.