P0301. Replaced coil & spark plug, now it's returned.

The best general approach for any kind of diagnostic data gathering is:

1. Nothing wrong with thinking out in advance what results you're expecting to see, and writing that down. That does help to focus you on what the data gathering can and can't tell you, and how you will recognize a surprising result.
2. Then you use the instruments and gather the data, following the ordinary procedure, not skipping bits where you think you know what you expect. You do the bits, and find out if they go the way you expect.
3. Then you compare what you got to what you thought you would get, and think about what that is telling you, and what you might need to know next.

 

Is P0301 still the only code you're seeing?

I experienced similar behavior after replacing the head gasket—the indicator didn't shoot to the extreme right, but it certainly seemed to be getting there faster and a lot more often for even modest accelerations with PWR mode off. I found two codes setting off the MIL—P0171 and P0142/0141 (probably P0142). Turned out I hadn't plugged in the EGR valve connector all the way, and I'd left out one of the bolts connecting the EGR pipe to the cooler, allowing air to leak into the fuel circuit.

By the way, early on in the thread some members recommended Gasket Masters. I would avoid them like the plague unless you have $1000 to gamble away. I'm ashamed to say I paid them much more than that for a head gasket replacement on my 2010, and they botched the job resulting in (what I later discovered was) a bent connecting rod, but could have been much worse. They ghosted me completely after I requested they honor their "1 year warranty". I subsequently replaced the head gasket, various other seals and gaskets around the engine, and the connecting rod myself, with genuine Toyota parts costing $317 in all, in my apartment's parking lot. Should you discover that it is indeed the head gasket causing the issue, you might go that route, or have a trusted mechanic or handy friend do it for you. Not Gasket Masters—look at their shady 5-star reviews on Google (all from January 2021, most reviewers having no other review activity). Look at their BBB reviews, which are less likely to be gamed.

 

They’re Priuschat members, I believe “gasketmasters” proceeded by an “@“...

not sure it’s wise to light that bat signal...

 

lets turn them in!

 

Your location reads Texas. Did the gasket masters do the job in CA, TX or somewhere else?

Asking because they were based in Nor Cal, but I've seen adverts in PA, NY, DE recently. Curious if it's the same crew or not.

 

It's the same outfit as the ones in the YouTube videos. They performed the job in TX.