Airworthines Directive about pressurization in certains models of 737

This AD was prompted by a report of a flightcrew not receiving an aural warning during a lack of cabin pressurization event. This AD requires incorporating design changes to improve the reliability of the cabin altitude warning system by installing a redundant cabin altitude pressure switch, replacing the aural warning module (AWM) with a new or reworked AWM, and changing certain wire bundles or connecting certain previously capped and stowed wires as necessary.

For certain airplanes, this AD also requires prior or concurrent incorporation of related design changes by modifying the instrument panels, installing light assemblies, modifying the wire bundles, and installing a new circuit breaker, as necessary. The FAA is issuing this AD to prevent the loss of cabin altitude warning, which could delay flightcrew recognition of a lack of cabin pressurization, and could result in incapacitation of the flightcrew due to hypoxia (a lack of oxygen in the body), and consequent loss of control of the airplane

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/0/54fe75ae13954c5d86257a8c004b3962/$FILE/2012-19-11.pdf