There are really only a couple options. You touched on one of them. Vane actuation is the easiest thing to check first, especially if you have access to a vacuum pump. If not, just let the motor idle and remove the vacuum line connected to the turbo actuator and watch the rod rise and fall as you repeatedly do it. It should move roughly 1/2" and do it fairly quickly and smoothly. If it takes more than about a second, or moves with a jerky motion, something is stuck. Usually running the engine hard on the highway with a lot of wide-open-throttle will burn out any buildup in the turbo. If it doesn't then a frustrating afternoon of taking the front half of the turbo off is in order. ALSO...check to make sure the lock nut on the actuator rod is snug. I recently had an overboost problem that was caused by a loose nut which I couldn't easily see from the top of the motor.
Check the turbo shaft for play to make sure the turbo isn't shot.
Cleaning the MAP sensor would also be a good thing to do before digging into anything mechanically.
If the vane actuation seems good and the MAP is clean, next would be to bypass the turbo vacuum modulator to see if you can force the opposite code. If it does, the modulator is fine. If it doesn't, then it needs to be replaced.
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