Got a P0139 code last week. Took her to the shop Monday. Wasn't the cat but the O2 sensor that needed replacing. Didn't cost much soo I was pretty happy.
Here's the thing, I decided to get my car inspected as the NJ inspection station was right next to the Mazda dealer and I was due by end of August. Thought I'd be fine with work just being done, but low and behold, I failed emmisions. The guy said looks like something had to be reset. I told him that I just had an O2 sensor installed at Mazda. He gave me a form for Mazda to fill out once they reset it. I immediately went back to Mazda just to have the tech tell me whenever you get emmisions work done you have to drive it 100 miles so it can reset itself. Told me to come back a fter 100 miles, they will stamp the form upon which I will pass inspection. Weird huh?

 

Usually whenever work is done to solve a trouble code, the tech will reset the computer in the car to erase the code. However this resets all the readiness monitors and checks that the computer performs. Had they known you were going to go and immediately get the car inspected they might have left the light on and suggested going in when it shuts itself off. But yea, drive it around for a few days then go again.