What is the sound of one piston slapping?
What is the sound of one piston slapping?

(Video placeholder for thumbnail because picture uploads are f’d for me)

So, I had my second ever idiot light this morning. First one was about 2 weeks ago. Both were P0420 codes, so presumably there really is something wrong since it came back. The facts:

- 2010 LH9 5.3L V8

- 125,000 KM

- I didn’t give it any warmup time this morning, just started up and left (but obviously didn’t beat on it cold). Don’t remember what I did the first time the light came on. I don’t see how this is an issue as I’ve done it at least 40-50% of my cold starts, but there it is.


- Both times the light came on in almost exactly the same place, about 3-4 minutes out of the driveway. And once cleared didn’t come back.

- The lights came in different tanks of gas (back to back)

- Could THIS be related? It died down through the summer, I also ran some of the additive recommended in that post through it. Now that the mornings are slightly colder it’s back. Dealer could not replicate in April.


- O2 sensor voltages - Upstream fluctuating as normal, downstream steady at ~0.745v. Both sides.

I know the primary cause of that code is a bad cat... 125,000km seems pretty early. What else should I check on?


EDIT: I also have this going on that I’m not sure what to do about. Could be a factor.

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