Boeing 737 operators have six years to modify cabin pressurization loss warning systems, including installing redundant pressure switches and new warning modules, FAA orders in an airworthiness directive (AD 2012-19-11) issued Wednesday.

TheAD, which applies to all 737 models, was prompted by a probe into an inflight cabin depressurization and passenger oxygen mask deployment. Boeing says the plane’s pilots did not receive an aural warning when cabin pressure exceeded 10,000 ft.; a cabin crew member informed them about the problem. The depressurization had nothing to do with the defective warning system, FAA and Boeing are careful to note.

FAA’s AD is based on service bulletins originally issued by Boeing in 2010. The changes include a second altitude warning switch, aural warning modules (AWM) and wire bundle modifications. “The installation of a second 10,000 ft. pressure switch and a new digital AWM with two fully independent channels will reduce the probability of the flight crew being not aware of a depressurization event,” Boeing explains in the bulletins.

FAA says the AD applies to 1,405 U.S.-registered airframes. Cost per aircraft is estimated at about $39,000, including $33,000 for required hardware. The compliance deadline is Nov. 7, 2018.