WASHINGTON — Boeing 737s will undergo more frequent testing of switches that are designed to warn pilots about a dangerous loss of cabin pressure.

Safety officials worry that if the switches fail, pilots could pass out from lack of oxygen. Planes are equipped with two cabin altitude pressure switches so there is a backup if one fails. However, airline pilots and maintenance crews are not alerted if a switch fails.

The Federal Aviation Administration posted an order Thursday that orders airlines to increase testing of switches used in the warning system on all Boeing 737s, including the Max, which recently returned to service.

The FAA says it's acting after getting reports of newly-installed switches failing.

The order does not remove any planes from service.

The FAA says the stepped-up testing affects about 2,500 planes registered in the U.S.