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Question: Check Engine Light, Will not pass inspection

Hello: I have a 2006 Caddy Escalade ESV. 6.0 liter AWD. My check engine light came on. A friend has a diagnostic computer tells me it's the gas cap. Replace the gas cap, reset the code. Back to the gas station, still will not pass inspection (Massachusetts). Put the computer on again, it reads canister purge valve. Replace that, reset the codes, still will NOT pass inspection. Kind of at wits end. Need the vehicle for work and cant' use it without a sticker. Also, and I think VERY important. When we reset the codes, the check engine light stays off, but still won't pass. When I refill the gas tank, it always starts hard. I actually have to pump the accelerator to help it along. Once it starts, it starts and runs great all the time. But the check engine light comes right back on. I'm thinking emission?? Vacum?? I have no idea, but will accept all suggestions..