Ping IAN : Code P0452

2001 Silverado 4.8L. SES Light is on. It's been reset. It returns. It is throwing code P0452. Gas cap has been replaced. Still throws the P0452 code.
Any ideas? Is it the pressure sensor in the tank? If so is it hard to replace? Any other possible causes?
any help would be greatly appreciated.
Thanks, Glenn
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