Cobalt p0420 code

I have a 2005 Chevy cobalt that is throwing this code. The check engine light came on a while ago and I replaced the ICM, the light still stayed on and I changed the oxygen senors. I turned the light off and drove around for a week the light came back on. I cleared the code again and drove around for a couple days then it threw the code again. The last time I started the car up it ran really really rough for 30 seconds and then it started to run smooth again. I took it to a shop and they told me that the cat was still good. Any suggestions?

I had to replace my cat when that happened. Is the shop certified?

Yes. The catalytic converter has gone bad. You may need to drop the downpipe and take a look in there. Most likely you need a new 400 cell cat.

hi

P0420
Catalyst System Low Efficiency