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My 2018 3350 was in the shop for 12 straight weeks last summer for this very issue. Kubota "reconditioned" cylinder head (small amount of oil getting past intake valve seals and valve guides causing "over fueling" condition). I'm skeptical. DPF and reformer also replaced under warranty. Bill would have been $6,000-$8,000 but for warranty.
Mine just regened (operating regen) successfully the first time a few days ago since I bought it a bit over 2 years ago. Total hours now 53.2, about 50% of which are in shop diagnostics, parked regens, etc.
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