Recently, I had a spark plug ejection event (rear center cylinder, #2 I think?). I was able to put in a save-a-thread insert and got everything back up and running. About a week later, the CEL came on with the P0420 code, which I never have had previously. The light went off after a few days, and then came on again a few weeks later before going off again after a few days. When I replaced the coil that was damaged in the event, I picked up the Denso one from Rock Auto that was marked as "Denso certified replacement" to try and save a little money. I also picked up the correct NGK plug from Rock Auto, because it was the only place I could find a plug for under $10 without buying a 4-pack or 6-pack. I'm not so worried about the plug, but could the fact that I didn't use the OEM coil be the cause for the intermittent CEL I'm getting? Looking for opinions here before dropping $170 on the OEM replacement coil. Thanks.

 

A P0420 is a catalyst low efficiency code.

The issue is with the catalyst. If the problem was due to a misfire or too much fuel going INTO the cylinder, the ECU would throw a code for something related to that. If there is some other issue, the ECU expects to see a rich signal after the cat...so it most likely won't throw a P0420. Or it will throw a P0420 along with other codes.

Codes are logic based as well as parameter based.

A P0420 by itself is just saying that everything else is working as it should. But despite all that, the catalyst's efficiency is low.

Look into a problem related to a catalytic converter. If you let the bad coil cause a misfire for too long, you may have damaged the catalyst in the process.

Now that you've fixed the coil, the ECU sees everything else is working fine...but the cat is still operating below its efficiency threshold.

Make sense?

 

Hitachi IGC0055 coil pack is OEM and it's $50.

https://www.rockauto.com/en/moreinfo...440495&pt=7060