2000 Suburban 5.3 lt

Po440 is to the evaporative system as a po300 is to a misfire. Maybe that not a perfect example, but essentially it means there is a problem in the EVAP system. It may or may not be a leak, it may be a purge solenoid that is bad or intermitantly bad, may be a cannister problem, may be a gas cap not on tight or leaking. A valve may not be sealing etc. Bottom line is the system is not achieving vacuum.

It's like anything else. It's like hunting deer. You become much more successful when you understnad them. :)

Maybe there are better suited filks that can help you out. Chris, Jack, Bruce and others here seem quite knowledgeable.