The FAA is asking airlines to inspect Boeing 737 switches that could pose a safety risk.

The Federal Aviation Administration on Thursday asked airlines to inspect a pair of cabin air pressure switches on all Boeing 737s, citing safety concerns.

If the switches fail, oxygen levels could drop dangerously inside an aircraft without warning. This could invalidate the flight crews, causing them to lose control of the aircraft.

“Resolving these failures requires immediate action,” the agency said in a directive.

The airlines did not report any failures that resulted in a dangerous drop in oxygen levels during the flight. But in September, an anonymous aircraft operator said the switches on three planes – all different 737 models – failed a test. Boeing decided late last year that the failures weren’t a safety concern, but the company and the FAA later concluded they posed a threat after further investigation and analysis.

The FAA guideline applies to all 737 models, including the struggling 737 Max, which was banned worldwide in March 2019 after two fatal crashes. This ban began to be lifted late last year and the Max has been used on thousands of flights since.

The order, known as the airworthiness directive, mandates a recommendation that Boeing issued to its customers last month, the company said in a statement. “Safety is our top priority and we fully support the leadership of the FAA,” Boeing said.

The order applies to approximately 2,500 aircraft in the United States and over 9,300 others around the world. Switches must be inspected within 2000 flight hours of the last time they were tested or within 90 days of the effective date of the order.