Ok so yesterday i hooked my little obd II scanner upto my truck and it gave me 6 codes p0300,p0420,p0430,p0452..it gave me 0420 and 0430 twice, so i replaced my valve cover gasket and today its giving me p0300 and p0452. Recently i replaced the fuel pump and everything so im not sure why its saying my fuel tank pressure sensor circuit low voltage and why it is saying i still have a randomn misfire do i need to replace all my spark plugs again since i did my valve cover gasket ive already replaced the wires the only thing i havent replaced yet is injectors which im trying to avoid replacing since they arent exactly cheap...any ideas on why it would be throwing any of these two codes

 

Replacing the valve covers will do nothing for the check engine light. PO 420 and 430 is catalyst efficiency below threshold bank 1 and 2. That is almost always a bad cat. The reason those codes have not came back is the catalyst monitor is most likely not on line yet. Does it run bad? If the cats are restricted it could cause a misfire. Po 300 may or may not be related to the PO420 and PO430 however the po452 is most likely unrelated as is is a different system