Ok so I've had this problem before and thought I fixed it but I guess not... I'm getting the P0497 (P0455) codes getting thrown up for the EVAP again. I had this issue before and replaced the solenoid up front and the CEL went away for a little while. It came back on so I went ahead and replaced the gas cap as well but that did nothing... I'm at a loss here on what to do.
Sounds like your in for an extended T/S condition here. Since you feel that the purge valve an gas cap are not the problem, your left with the possibility of a faulty tank pressure sensor, bad EVAP canister, and or its vent valve, leak in the purge line from the engine back to the tank, or that same line is plugged. Any of which can prevent the correct level of vacuum getting back to the tank in a timely manner. Or the vacuum is unable to hold at the required level once the supply from the engine (thru the purge valve) has timed out an shut down the vacuum source. You may have to settle for the GM service department on this, an have them run the complete EVAP system test to locate the problem(s).
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