Boeing has recommended that airlines temporarily take the 737 Max out of service because of an issue with the plane’s electrical systems. It’s the latest in a now very long line of problems that have cropped up with this particular plane since two of them crashed in 2018 and 2019, killing 346 people. But it’s the first to ground the plane since the Federal Aviation Administration approved its return and it went back into service late last year.

The new problem has to do with whether there is “a sufficient ground path” for a particular component of the plane’s electrical power system, according to Boeing. Southwest Airlines, which is the most dependent in the US on the 737 Max, has already grounded 30 of the planes, while American Airlines has pulled 17 and United Airlines has pulled 16, according to Reuters. This all comes as air travel is rebounding in a big way in the US, thanks to a dramatic increase in vaccinations against the coronavirus.

That’s a different issue from the one that led to the two fatal crashes, which involved Boeing installing a piece of safety software that it didn’t tell pilots or the government about as part of a rush to bring the plane to market. But it’s not the only one to come to light since those crashes. Boeing previously found debris in the fuel tanks of 737 Max aircraft, for example.

The FAA — which was accused in December by a Senate investigation of working with Boeing to manipulate tests during the recertification process of the 737 Max — said in a statement on Friday that it “will ensure the issue is addressed.”