They sure do fail randomly. I drove to the lake one day last July, when I got home, reverse was just about gone and I had a weird swishing noise. Reverse band was blown, so I had virtually no reverse and a very unreliable 4th gear. It kept getting worse over the next 100ish miles until it was having problems shifting through any gear. I swapped it for a low miles used unit, which, 7k later, is still working great!

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)