Errr....Ifixplanes...does that mean manually selected to HIGH, or auto selection to HIGH as a result of turning off the other pack?
Also, when the pressurization system has the cabin in excess of 2.5 psi (when the overboard exhaust valve is normally closed), the NG will partially open that valve, regardless of the capability of the system to maintain proper pressurization.
My understanding is that the classic will only open that valve if pressure can be maintained.
Cheers, FD
