Oldsmobile/Buick Repair: Emissions Control System, emissions control, rateing

Question
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Followup To Mr. MCMillen a dealership told me that there is a part located on top of the gas tank, a valve that maybe bad or that a electrical connection under the driverside maybe corroded, to clean it and reset the computer.  Could this be true I would have to bring it in for $40.00 to scan.  I trust your word more.  Thanks Dave.
Question -
Dear Mr. McMillen I have a 1999 Olds Silouette Van with a 6 cyl. 3.4 and have had the check engine light come on.  I had it coded and the problem is a code PO440 Evap. Emissions Control System---PO449 Vent Value-Sol Mal I am a shade tree mach. could you tell me a little more about what maybe wrong and can I buy this part and do it myself.  Thanks a lot.  Dave.
Answer -
hi dave,well the evap system is basicly designed to pressurize the fuel tank briefly if it can,t pressureize at anytime it will set a code .what can you do tighten your gas cap or replace the cap.if the cap is ever left loose after putting gas in it the code will appear.now there is other parts of this system but are coded as such.remove the neg cable be sure cap is tight and this should take care of it .please take a min to rate my help lol most don,t ty billymac

Answer
HI DAVID.YES THIS COULD BE TRUE ,AND LIKE I SAID NORMALLY IT IS THE GAS CAP LEFT LOOSE .BUT NOT ALWAYS .IF IT IS NOT THIS THEN YES THIS IS WHAT I WAS TALKING ABOUT WHEN I SAID THERE IS OTHER CODES AND TEST TO DETERMINE IF IT IS A SENSOR ON THE TANK OR A BAD CONNECTION IN THE WIRE.MY RECCOMENDATION IS THE SAME.DISCONNECT THE NEG CABLE FOR 5 MINS OR SO TIGHTEN CAP NOT EXCESSIVELY JUST NORMAL.IF CODE COMES BACK YOU WILL NEED TO HAVE IT SCANNED.SO YES THE DEALER IS RIGHT AND MOST OF THE TIME THEY ARE BUT IF YOU CAN TAKE CARE OF THE PROB WITHOUT THEM YOU WILL BE 40 AHEAD LOL GOOD LUCK TY FOR RATEING ME BILLYMAC