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This deals with the purge solenoid, vent solenoid, charcoal canister, all the vacuum lines in this system, as well as the gas cap. Any one of these components may contribute to this code being set. All the wiring should be tested including the connectors for any problems. Vacuum lines should be checked for cracks or loose connections. The canister may be damaged or not working properly.

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Most likely the canister purge valve or vacum valve is stuck open allowing engine vacum into the fuel tank.
Also if you have had any work done and the vacum lines were installed backwards, this will also cause your problem.