I have a MSX150 that is constantly setiing off over boost fault code.
Put it on the water and pull the throttle, engine check light comes on instantly.
Switch off and back on it will rev up but fault again is displayed and rpm limited.
So far I have done the following

Intercooler was coated in milky oil. Washed using kerosene and hot air dried. Intercoller is now spotless clean and dry.
I did have a small problem with the engine overheating after intercooler cleaning,mall amount of kero remained and was causing the engine to run hotter. another hour of drying and problem sorted.

Removed oil tank, disassembled and cleaned.

I replaced the 2 pressure sensors.
Checked banjo fitting for blockage, all good there.Replaced seals on banjo fitting,
Checked operation of wastegate solenoid, working good
Wastegate is moving freely, not sticking.
I checked the wastegate actuator lift off pressure, it was slightly high at approx 8psi, I adjusted it to lift at 6psi.
I also pressure tested the hoses on the solenoid, just in case one was leasking.
Had the codes reset using DW. Only code showing is "overboost condition"

What else might be causing the overboost problem. It is there from the very first pull of the throttle.