I was driving home from lunch just now and my 13 SH threw the code. I've had my j pipe and catback on for months. has anyone had this code before?

 

Best case scenario you have a leak, probably at the aftermarket j-pipe where it connects with the rear cylinder bank catalytic converter. The p0420 is for the rear bank. Worse case scenario...the rear cat has collapsed (doubt it unless you messed with the rear cat too). So check for pin holes where the rear cat connects with the j-pipe. Any rust there?

 

no, there was no rust. i just got into my car an hour ago and it magically didn't come on again...? i didn't do anything to it. i just looked for rust and cracks.

 

I recall in the older J engines (like j30a4), the procedure to force the light to come on again, if there is indeed a catalytic issue, is to: After you reset and turn the light off, do a couple of long decelerations with gear in, careful with traffic, highway off-ramps are ideal for that. Then shut down and let it cool for a few minutes. Then get on a freeway again at constant speed. This will force the computer to start doing its self-testing for emissions. If all goes well then no light. But if it detects that the upstream and downstream sensor readings on the rear cylinder bank catalytic converter are not within spec then it will turn on the engine light. To test further if the light turns on, decelerate again in "N" a couple of times, let it cool down and then without decelerating in-gear again try to get on a freeway at constant speed. If it turns off, your cat has collapsed inside. If it doesn't turn off then its the O2 sensors (most likely the bottom one) that has gone bad. It also helps to have an OBD reader to see other readings that can help diagnose this properly.

 

The emissions CEL just went away by itself after two days..?