99 Toyota Solara V6: P0300, P0301-P0306, P0141, P0446

Friend's Solara, trying to figure out whats going on. Car stumbles and has no power.

Throwing a host of codes, P0300, P0301, P0302, P0304, P0305, P0306 (all misfires except cylinder 3). also throwing P0141 (O2 sensor pre-cat) and P0446 (electrical circuit for the vent control on Evap system).

Kinda at a loss as to what is going on... where are some good places to start?

Your P0141 is actually heater circuit malfunction, bank 1 sensor 2. This is the post catalyst sensor which monitors catalyst performance. This sensor probably needs replacement. For the misfire codes, I would start with checking the basics. How long has it been since the spark plugs were replaced? Are they the correct type and brand? If they are not the correct Denso or NGK plugs then they are likely the cause of your problems. Are any vacuum hoses disconnected or leaking? Are your air filter and pcv valve clean and correctly installed? Your PO446 is likely to be one of your vaccuum switching valves, either on the charcoal canister or the back of the air cleaner housing. I wish I could tell you exactly which one, but its now been more than a year since I left the dealership.

Thanks for the reply...

The plugs are NGKs and correct to the car, also recently changed. Air filter, PCV and vacuum hoses all check out fine...

I'm not overly concerned with correcting any evap or mpg issues currently, just trying to get to the bottom of the stumbling and lack of power...

Check your basics first, like Fuel pressure, i would do this first then when that's OK i would check engine compression.
fuel pressure will give you the mentioned codes.

what is the FP supposed to be?

I should also share a little background... its been doing this intermittenly, and still somewhat intermittenly but with an increased frequency, for about 6 months or longer.... (which most likely eliminates bad gas). It also had a fuel injector leak at the o-ring which was what I thought was causing it, but after repairing, the symptoms came back.