I have a 2007 Toyota Corolla LE, with about 47k miles on it. Getting P0300, P0301, P0302, P0303, P0304, and P0353. Basically, misfiring all four cylinders and an ignition coil (actually I'm not sure if the other coils also showed an error). The spark plugs have never been replaced from the originals.

I could have water in the gas, or something else could have gone bad... not sure what to do. Here's the background:

Went on a roadtrip recently, and stopped at a gas station to fill the tank with what was labbeled 87 octane gas. Soon after getting on the highway, the check engine light came on, and the engine began to occasionally misfire.

Figured maybe it was bad gas, so I kept driving a few hours until half the tank was empty, and put in a bottle of STP concentrated gas treatment, followed by 93 octane gas from an Exxon, which I assume was good.

The engine still misfired and the check engine light stayed on as I continued the drive home, about 300 miles.

Checked the connectors for the ignition coils and spark plugs to see if they were loose, and they seem fine. I figure its unlikely that all four spark plugs and/or ignition coils would go bad at once.

The mechanic said it would be a pain to drain the gas tank, and I called the dealer, who said my car is not under warranty for this anymore. They said to come in for a $94 test to see what is wrong, could be the injectors or spark plugs, or the computer, or the... list goes on.

What I do in this situation is cut off the gas supply and hook a can of injector pressurized cleaner up to the fuel rail. This will prove bad fuel supply if the misfiring stops.
You would have to pay the 94$ for that service, have it diagnosed, and get your injectors cleaned all in one shot.
If contaminated fuel is the problem, for an additional cost, the dealer can drain your tank with the Toyota scan tool running the fuel pump to drain it.
Not really a big deal.

So, I went to the dealer and had them run the diagnostic tests, and they told me they think the 3rd Ignition Coil is bad. This was based on the ODB code being 353, for Ignition Coil C being bad, and they moved that coil from one cylinder to another, and could follow the code to that cylinder. They recommended I change the 3rd coil, and also have the spark plugs cleaned and regapped.

Except, I don't know if I believe them. I opted to have my mechanic swap out the coils, because Toyota was going to charge like $300. Got the coil from Advanced Auto Parts, and we changed it, but the car still misses. The check engine light came back on, and although I haven't checked the codes yet, I have a feeling they will still be 300, 301-304, and 353.

I can go back to the Toyota dealer Tuesday(3 days from now) and they will see what went wrong with the diagnostic test.

Should I try replacing the spark plugs, or throw some fuel injector cleaner in there? Or should I leave it to the Toyota mechanics?