My second head gasket (relatives car) didn't go well

I helped a relative do his head gasket on a 2010 two weeks after I did mine. Everything went well except for the timing was off. So I cracked it back open and did it right. I admit to not watching closely when I put the crank pulley back on and turning it two revolutions. I was apparently off. The second time was the charm.

Except for a flashing check engine light and the following codes-p304 and p0420. The car sat then a couple of weeks later I was able to take a look. We swapped coils, even swapping spark plugs, and even swapped a couple of fuel injectors (a suggestion I read somewhere).

Never have had the check engine light come on again, and at some point the p304 code went away- but never the 420. It's running a little rough at say 30 mph plus and a we are getting misfires (as reported via Car Scanner app) on cylinders 2,3,4. At one point it was 4 only.

I did not now until we were finished- the driver had already been told "you need a catalytic converter". I never drove it before we did the head gasket and I never knew what codes it had before we did the head gasket. It certainly did need the job done-that was advised by the shop, but the customer declined.

I love a little help from AI- based on what we've done so far, she is suggesting:
Summary of Next Steps:

1. Prioritize the Exhaust: Crack the exhaust flange open and test drive (or at least rev the engine) to see if the sluggishness disappears.
2. Verify the Cat: If the P420 was there before the head gasket failure, the coolant that leaked probably finished off an already aging converter.

Would the fine scholars and engineers here at Priuschat recommend the same steps or some other course of action?


Some of you might remember I did a successful head gasket replacement on my 2014, totaled the car 3 days later... I just purchased this week my Prius 2.0- a 2019 that is an a very stripped down fleet model, but has all the goodies I really wanted- adaptive cruise, collision avoidance, blind spot, auto dimming headlights, and the parallel parking thing!

 

Help me diagnose the misfires. I can probably investigate how old the spark plugs are, the coils look new. I’m told both are fairly new.