P0300, some thoughts:

[Brian in Tucson]

I hooked up the Durametric for the first time and pulled a P0300 and the associated codes: P0301 thru P0306. Multiple misfires. 01 Boxster base. I've been working on this one forever. Newer low mileage factory rebuilt motor, new plugs, coils, air filter. After I put the engine in it ran well for a time. Durametric says the cats might be damaged. The engine was backfiring into the exhaust, but I doubt the cats were damaged--we shall see I guess. I have a set of new O2 sensors, all 4.

Side notes, the car's been sitting for about a year. Gas probably is not the best--it was originally Chevron Premium. It's got a dose of octane booster. And the tank is 1/3 full.

It's been awhile, so my memory might be a bit faded. I think the problem came on rather quickly. It was backfiring into the exhaust, but before the muffler. No boom! out the tailpipe.

The P0300 code combined with the new ignition components leads me to think it's the fuel filter is clogged. Makes for a really lean fuel mix. And with the tired gas, it doesn't run at all.

I ordered a new filter today, but I'd welcome any alternative theories. While I'm waiting for the part, I'll drain the remainder of the gas out. Always lots to do on the 'cheap' 01 Boxster.