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.