B737NG: High Cabin Diff Pressure

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When we had this problem, we put two snags in the logbook:
1) Failure of the CPC (and failure of the CPC to recognize its fault)
2) Failure of the AUTO FAIL light to illuminate above 8.75 PSI which was clearly the case here.

So maintenance had to find out why these two seemingly distinct problems occurred simultaneously.

It turns out that the real culprit was the failure of one of the Outflow Valve's two Cabin Pressure Switches.

"The cabin pressure switch located in the outflow valve is set to actuate when the cabin altitude exceeds 14,500 feet (8.46 PSIA) and will cause the outflow valve to close independent of the control system. "

".... the cabin altitude at the time the switch closed was much less than 14,500 feet ... the outflow valve closed unknown to the crew and the cabin altitude decreased until the differential pressure exceeded the cabin pressure relief valve threshold'"

When a true depressurization occurs, the sequences of faults in the CPC are:
FC17 (CAB_ALT_10000_FT)
FC18 (CAB_ALT_13,500_FT)
and last FC90 (CAB_PRES_SW_ACTIV) at 14,500 feet but only of no crew action is taken.

There is a cabin pressure switch located in each of the two Eboxes of the outflow valve, but only the active one controls the cabin and the outflow valve but both CPCs will independently record these faults in their memories.

If a FC90 fault occurs only in the active Ebox, the outflow valve will be commanded to close and the AUTO FAIL light will remain extinguished.

A software change (Nord Micro Service Bulletin CSB 21933-21-003) with case the AUTO FAIL light to illuminate in case of the failure of the outflow valve pressure switch).

As a final note, the cabin pressure switch that provides cabin altitude to the gauge in the overhead panel is obviously distinct from the one that controls the cabin and the outflow valve......