Vent valve is on intake manifold gets broken occsionaly on intake gasket repairs I've replaced in 5 minutes a simple repair but check hoses for leaks first also at check dealer I smoke tested many times u can use tech 2 command purge valve increases while watching fuel tank pressure from ftps fuel tank pressure sensor readings if it falls off smoke test for leaks found various places necks. Tanks .lines .purge as well as vent valves needs to be checked by proffesional and sadly most techs don't have the knowledge even at gm shops

 

Here you go.
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Yup I changed that and so far no evap code finger closed that it last until the 10th

 

Glad to hear it. Good luck.