If you have 'overboost' limp mode, it is most likely your VNT mechanism sticking (turbo variable geometry/internal wastegate)
The moving vanes/ring get jammed up with crud and stop pivoting smoothly, if the mechanism 'catches' or sticks for a few seconds, your MAP will register an overboost (or underboost).
A good long run carrying a lot of weight can temporarily cure the problem (high turbo heat), but generally cleaning the turbo, either chemically or by physically dismantling will solve the problem.
If you take the problem to a dealer, they will simply try and replace the complete turbo unit, as they do not class the turbo as a serviceable item.
Other areas to look at are MAF, vac control leaks etc etc, all the usual 'limp mode' suspects, these will however generally cause underboost fault codes.
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