Boeing Co. 737s that have been put in storage during this year’s pandemic are at risk of losing power in both engines and must be inspected, aviation regulators said in an emergency order.

The order applies to Next Generation and Classic 737s, two generations of the model that is the most common single-aisle jetliner in the world. The Federal Aviation Administration said in the order dated Thursday that it was prompted by four instances in which a corroded valve caused one engine to shut down.

Airlines around the world have grounded thousands of jets as the air-travel market collapsed due to the Covid-19 pandemic. There are multiple steps carriers must take after a plane is put into storage, but the order suggests that the corrosion issue wasn’t previously known.

The order applies to about 2,000 jets in the U.S. alone. While FAA doesn’t have authority in other countries, such orders are almost always adopted by other regulators.

The order doesn’t apply to the 737 Max, the latest version of the 737 that has been grounded for a separate issue since March 2019.