A friend of mine had the same symptoms and it ended up being a broken camshaft. Low mileage diesel in a sprinter.

I agree, the mechanic should have done a deeper inspection and shown a cause before using the general term “locked up”.

Many things can cause a locked up situation. Some external like a seized alternator or other driven accessory.

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Satan creates nothing: he only ruins everything. He does not invent: he tampers. And his followers are no different ~ Archbishop Carlo Maria Viganò