When I was managing M&R for a large trucking company - our issues with DEF seemed to be 2 things.
1. Operator error. Drivers not adding it, adding it to fuel, adding diesel to DEF tank etc.
2. Corrosion of the electronics associated with it. Freightliner really didn't want to talk about that one but we'd get them covered (ordering 4500 trucks a year from them gets attention)
