most common issue fix for this problem is the n80 valve getting plugged from charcoal pieces from the charcoal canister. take the n80 valve off and tap it on a white piece of paper. if you see any black particles on the paper, you need to replace both your charcoal canister and your n80 valve. be sure and blow out the evap lines with compressed air if this is the problem. when you have the n80 valve out try applying vacuum to each end it should hold vacuum at both ends. Testing the evap system via basic settings is also a good idea. To ensure that the gas cap isn't the issue.