Boeing announced a new problem with the troubled 737 Max -- this time, the issue is with its electrical system.

"Boeing has recommended to 16 customers that they address a potential electrical issue in a specific group of 737 Max airplanes prior to further operations," the company said. 

Airlines need to verify that one of the components of the plane's electrical system is sufficiently grounded, Boeing stated. The company said it is working with the US Federal Aviation Administration to address the problem.

All of the planes were grounded for 20 months worldwide from March 2019 through November of 2020 following two fatal crashes that killed 346 people. Although the FAA has approved the fixes that Boeing made to the malfunctioning safety system blamed for the crashes, several other countries, including China, have yet to clear the plane to fly again.

The grounding cost Boeingmore than $20 billion. Boeing (BA) did not identify which 16 airlines are affected by this alert, or how many of the planes are affected by the problem. Many airlines that own the planes have been eager to get them back in service despite having many of their planes still parked during a period of reduced air traffic caused by the Covid-19 pandemic.

The planes are more fuel-efficient and have lower maintenance costs than earlier versions of the 737, which have remained in service during the grounding.

After a year of very weak demand for air travel, air traffic has picked up in recent months, especially in the United States. The airlines do not need to cancel flights to replace planes in their fleets.

Four US airlines -- Southwest (LUV), United (UAL), American (AAL) and Alaska Air (ALK) -- have the 737 Max planes in their fleets and are currently flying them. 

Southwest and United have placed orders for additional 737 Max jets since the grounding was lifted. Southwest said Friday that the issue affects 30 of the 58 737 Max jets in its fleet.

It says those planes will be replaced by other aircraft and that it "anticipates minimal disruption to our operation" since it flies Maxes on a limited schedule and the planes account for only a fraction of its planes in service.

American, the first US airline to begin flights with the Max in December, said last month that it has not seen any indication that passengers are less willing to fly the Max than they are other jets.