Chevrolet Repair: OBD2 emissions codes, vent hoses, chevy avalanche

Question
I have a 2006 Chevy Avalanche 5.3l V8. I recently failed state emissions with codes p0300 & p0446. My vehicle has never run rough and two weeks ago I replaced the spark plugs & wires, had fuel injection service done and relaced the fuel cap and the "evac emissions control vent valve solenoid". I am a loss for what to do next. Please offer come suggestions.   Thank you.

Answer
HI STEVEN, WELL THIS IS A GOOD QUESTION BECAUSE IF YOU DO NOT HAVE A SCAN TOOL WE ARE ONLY GUESSING..WHAT YOU REALLY NEED TO DO IS GET A GM SCAN TOOL OR SIMILAR TOOL [GM SOFTWARE] TO RUN A FUNCTION TEST ON THE EVAP SYSTEM... TURNING THINGS ON AND OFF TO SEE WHAT IS OR IS NOT WORKING..THE PO 446 CODE IS FOR THE EVAP VENT VALVE SOLENOID SO THAT IS WHY YOU REPLACED THIS,BUT THERE ARE OTHER THINGS THAT CONTROL THIS SUCH AS A PURGE SOLENOID FOR ONE.. ALSO DID YOU MAKE SURE THE HOSE GOING TO THIS WAS CLEAN OF ANY DEBRIS ??? THESE VENT HOSES GET DIRTY AND WILL BLOCK THE FLOW TO THE VENT SOLENOID..SO YOU COULD CK THE HOSE GOING TO IT AND GM DOES HAVE A NEW UPDATED HOSE AND FILTER AND ROUTING TO THIS VALVE...IF ALL THIS LOOKS OK I WOULD URGE YOU TO TAKE THIS TO A SHOP THAT CAN RUN A DATA FLOW SCAN TO GET TO THE BOTTOM OF THIS..OR YOU WILL BE JUST THROWING PARTS AT THIS..
AS FAR AS THE PO300 THIS IS FOR A RANDOM MISFIRE CODE. WHICH BY THE SOUNDS OF THINGS YOU ALREADY KNEW THAT AS WELL. SO IF ALL SPARK PLUGS AND IGN WIRES ARE NEW,YOU ASK, WHAT ELSE IS LEFT.. IGNITION COILS ??? VALVE TRAIN ISSUES,SUCH AS BAD GUIDES...NOT VERY COMMON BUT WE HAVE SEEN THIS. DO YOU HAVE OVER 100K ON THIS ENG ??? IF YOU HAD A SCAN TOOL AGAIN YOU COULD CK FOR MISFIRE COUNTS AND ZERO IN ON WHICH CYLINDER OR CYLINDERS ARE CAUSING THIS SKIP..SO THIS MAYBE CHEAPER FOR YOU IN THE LONG RUN TO GET THIS SCANNED AT A GM SHOP THAT KNOWS HOW TO RUN THERE SCAN TOOL TO ZERO IN ON IT FOR YOU..THAT WAY IF YOU WANT TO INSTALL THE PARTS THIS WOULD SAVE YOU SOME CASH..AS FAR AS PASSING THE EMISSISON TEST,I WOULD FIX THE EVAP CIRCUIT PROBLEM FIRST AND THEN CLEAR THE MISFIRE CODE AND DRIVE IT FOR ABOUT 50 OR 60 MILES OR SO [ EASY ] NO BIG HILL CLIMBING] TO NOT SET THE MISFIRE CODE AGAIN AND GET IT INSPECTED. DRIVING IT FOR 50 MILES OR SO WILL GET THE MONITORS READY TO PASS INSP AND SHOULD NOT RESET THE CK ENG LIGHT..I WISH I COULD BE OF MORE HELP TO YOU BUT WITHOUT BEING THERE TO DO A HANDS ON SCAN IT IS KIND OF HARD. LET ME KNOW HOW THIS GOES AND GOOD LUCK TO YOU,,,TOM..