The other night, my 2011 5.0 (w/ 175K) started misfiring pretty erratically. I changed out the plugs and swapped the COP's. Did a compression test and all checked out ok. Took it for a drive and barely made it home. Accelerating was minimal and threw the following codes:

• P0300
• P0301
• P0302
• P0303
• P0304
• P0306

I also noticed P0420 (catalyst system efficiency below threshold bank 1). Any thoughts because I'm at a loss....

 

so basically your misfires likely caused the p0420. That error code is downstream o2 sensor related. Basically says hey you arent cleaning up the air with this cat and well in your case you weren't. You were dumping raw fuel into the cat by not firing it (misfires) if you continue to misfire and continue to send raw fuel (which I don't think you will your probably going to fix that misfire) andy dumpling raw fuel to the cat will eventually burn it out permanently does that make sense?