Hi, I'm new to the forum and have a question regarding the check engine light. It has been on for some time now. I went for my emissions test and they saw the light, plug in the tester and stated there was a PO 420 code referring to the catalyst system. I went to the local Autozone and used there tester, which came up no codes. The light went off on the way home. Do I take it back for the emissions test (1st one failed) or could it be something else?