Chevrolet Repair: OBD II fault code, obd ii, vent valve

Question
I have an OBD II fault code P0455 coming up on a 2007 impala 3.5 L engine Flex Fuel. Was told to replace the gas cap. Went to the parts store and replaced the gas cap which is yellow. After clearing the code it came back. Same code. Do you have any idea what this code is and what part might be bad. Was also told that the Evap. Emission Vent Valve might be bad. Do you know where this might be located and how you would check this to see if it is bad. Or do I have to take it to a garage to have it checked. Don't want to have to do that. Thanks.

Answer
HI BOB,I JUST REPLACED A VENT VALVE ON A 2006 IMPALA LAST WEEK. THESE DO HAVE A HIGH FAILURE RATE...TO TEST IT YOU COULD JUST SUPPLY POWER AND GROUND TO IT AND LISTEN FOR A CLICKING SOUND WHEN YOU ACTUATE IT,IF NO SOUND IS HEARD THE VALVE IS BAD..THE CORRECT WAY TO DO THIS IS TO GO THROUGH THE PCM WITH A SCAN TOOL AND DEMAND IT TO COME ON. THIS WAY YOU ARE TESTING THE PCM AND WIRING..BUT IT SOUNDS LIKE YOU DO NOT AHVE A SCAN TOOL.SO IF YOU WANT TO GUESS AND HOPE ALL ELSE IS OK,, THE VENT VALVE IS NEXT TO THE GAS TANK FILLER NECK UNDER THE QUARTER PANEL COVER SHIELD..BY THE WAY, THIS CODE IS FOR A GROSS EVAP LEAK. WHICH COULD BE FROM THIS VALVE HOLDING OPEN..  I HOPE THIS HELPS YOU ,,,TOM..