The best car to buy is not the fastest, cheapest, or most beautiful, and you won’t find the most important spec on paper. Sure, it can’t be too expensive, too slow, too ugly, or too costly at the pump. But more important than any of those, the best car to buy is the one that just works. When you turn the key, the engine comes to life, and you get to where you’re going. Cars symbolize freedom because they let you go wherever you want, whenever you choose. If your beautiful Alfa Romeo Spider is dead in the driveway, though, it’s failed that duty. That’s why Toyotas are so popular.

I’ve been thinking about this as I use Sennheiser’s flagship wireless earbuds, Momentum True Wireless 4. Like many critics, I thought they were among the very best on the market. As I wrote in my review for these pages, they are expensive, but they combined top noise-canceling with excellent sound quality, great size, and great ease to use. Put simply, I thought they were the best balance of everything you’d want in a pair of earbuds, and said that, if you were willing to spend the money, I couldn’t recommend them more strongly.

But with their reliability issues over the past several months, I have to retract that. However much I love the sound profile, I cannot recommend you buy these earbuds because mine haven’t worked.

The first long-term issue was an obvious problem at the announcement: the color of the fabric-covered charging case. Initially a clean light gray, it has since absorbed a lot of everyday detritus, leaving it a slightly browner, uneven shade that conflicts with its supposed premium image. Is that a deal breaker? No, but I wish they had at least offered a case made from a different material. All my attempts to clean the fabric have proven unsuccessful.

The bigger issue, though, is that my first pair of Momentum 4s, and the replacement pair I was sent, failed due to the same issue: a physical failure in the right earbud. After a month or so with the first pair, the right bud made a severe screeching noise when touched or moved slightly. They were replaced, but less than two months later, the replacement earbuds developed a similar issue. The right earbud won’t play audio unless it’s sitting in a specific position in the ear, with sound cutting out in most normal, comfortable positions.

Neither set had ever dropped or been near water, and I keep the tips clean. This issue has never happened with any other earbuds I’ve owned. Upon inspecting the second faulty bud, it seems the glue inside had softened, causing the drivers and other components to move during use. This would explain the squeaking. Though I was told in May that they hadn’t heard of this issue, two failures of the same kind in a row, both likely due to glue strength, suggest this is not isolated.

This doesn’t mean you should forsake Sennheiser as a whole, just this product. Their Momentum Sport gym earbuds have been flawless, and their only issue is their steep price and incoming competition in next year’s Powerbeats Pro 2.