I have a 1996 gmc truck 4.3 v6 I replace my spakplugs and wires the coolant temp sensor. check engine light went off ran good for a couple of days now light is back on and truck doesn't want to start than when it does it runs really ruff till it is warmed up. any help would be welcomed

read out p0300 says ignition system fault-plugs wires or coil; vacuum leak injector fault or high or low fuel pressure..p0420 says catalytic converter defective failure possible due to number 2 3 or 4 engine misfire or running condidtion large vacuum leak seals, piston rings

update.. have not fixed anything yet.. I left my battery un hooked for about an hour hooked it up turned the key and it fired up on first try.. so I don't know what that means.. don't know if I checked spark wrong is why thought it had no spark or if the reset fixed that... does this mean my mass air flow sensor is bad

or coud it be ignition coil or the module???