what's the possible cause for no engine power and code p0455 ?

Question

06' pontiac g6 engine code p0455,p0300- engine has no power to accelerate even in park w/ noload new gas cap

Answer 1

P0455 Evaporative emission (EVAP) system large leak

When P0455 occurs it uses the FTP (fuel tank pressure sensor) to determine any EVAP leaks. It the FTP was shorting out yes it can affect the MAP (manifold absolute pressure) sensor 5 vot refference that it shares with it, If this was the case you would have a P0106, But if you only have a P0455 without a MAP sensor code then No. Since you have a P0300  (random misifre) this would be the cause for no power to accelerate.