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.

sound like a nightmare problem..Thanks

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.