Bad gas, or something else?

Filled the tank about 3 weeks ago and within 2 miles of the station I got about a 15-second episode of misfiring, feeling like it was running on 5 cylinders, with a flashing CEL. It then cleared up and drove fine. Got home and had 2 codes: P0300 Misfire damaging to cat. converter P0305 Cylinder 5 misfire damaging to cat. converter I cleared the code and swapped coils 5 and 6, since they're easy to reach, figuring if coil #5 was going bad and creating a misfire, I'd see the misfire move to cylinder #6. In doing this I could see both coils 5 and 6 had some superficial cracks, so I figured maybe at 90k miles it's time to replace them all. The spark plugs were all replaced at about 70k, so I don't think those should need replacement. All 4 oxygen sensors are also new-ish, probably replaced around 80k miles. Car drove fine for the next week or two. Figuring it could be bad gas, I refilled from a different station when it got down to about 1/2 tank. I also installed new updated 997 Beru coil packs as a preventive fix, finding all 6 originals to have some minor cracks here and there. After about a week of those, I got a steady CEL (not flashing) P0420 code, catalytic converter-related. The car never misfired or misbehaved in any way. Cleared the code and drove for another few days, and the P0420 came back. I did tap on the cats with a rubber mallet and I don't hear anything rattling inside. I've still only burned maybe 2/3 of a tank of gas since the original fill-up that seemed to trigger that short bout of misfiring. My commute is really short, so this is all been in-town, slow driving; haven't had a chance to really take it out for a long or aggressive drive to get everything good and hot. I have a track day in a week and a half that will be hours of highway driving to get there and then hours of track time, so I'll certainly empty the tank a few times and get everything up to steady hot temps for a long time. Since the misfire has never returned, the car runs as well as ever according to my butt dyno, and the P0420 seems to just indicate poor catalyst performance and not necessarily something that will damage the engine, my tendency is to just keep running it and see if it continues. I'm not going to throw a new converter at it until I have more proof that it's truly the issue. Anybody got experience or ideas?

I've had a similar event on a honda,, load of bad gas, I think the misfires basically sooted up the cat and then the post cat 02 got dirty as well. I ran a full dose of injector cleaner (Seafoam) and topped off with fresh fuel, Did the cleaner for 2 full tanks and the problems cleared up. I did use my borescope to look in the cat by going through the O2 sensor bung and the cat was totally covered in soot like a fireplace chimney. I brushed the o2 clean , blew it out with some air and hit it with a small propane torch. About a year later the O2 died. I's assume Techron would probably do similar.

I've had bad gas, but never had the urge to stick a borescope up my bung...

If push comes to shove with the Converters: https://www.ebay.com/i/200979148511?...QaAhVYEALw_wcB

Make a claim with the gas station, there may be others like you out there, and they are already aware of it??

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Got experience with that part? Gotta be a reason the prices on these things vary by a factor of 10 A lot of people seem to suggest the aftermarket ones have less material in them, so they don't last as long. I have no data either way.

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Not with that specific part...but with other cats on other vehicles....I would not spend 3K on cats on a 20 year old car...if they last 5 years...then I replace again.....it would take 5 more replacements (another 25 years) to make up for the 3K

1 Unscientific thought,, I've run a 200Cell cat on my 94 Jeep since 1998, It still passes with flying colors.. I went through 4 stock cats before the Random Tech cat. YMMV!

Status update: after a ~400-mile highway drive today, no CEL, and no fault codes showing at the end of the drive. I dumped a can of Seafoam in before the trip just for fun, so that has been consumed. 2 days at the track coming up, will report back any gremlins that pop up. All signs are pointing to bad gas from a particular Sheetz station, but will be holding my breath for a few more fuel runs before I feel confident that was the cause.

Further status update: after a few more tanks of gas consumed on track and on the ~400-mile highway journey back home, still no CEL returning. All signs are pointing to a lousy tank of fuel.