I tried the diagnostic activation within FiatECUScan and as with your test, the activator moved back and forth as you would expect. However, if there is a residual resistance in the solenoid circuit, the solenoid will not be opening and closing at the correct time.
I also had the P0236 error (the underboost one)and if I recall, the message actually says that there is "unexpected voltage received by the sensor."
For me, the bad connection was at that multi connection plug to the right of the rad. I carefully exposed a section of the cable on both sides of the connector. On the correct cable, (be careful of the colour coding as there are a couple that look similar and with time, the colour changes) at the ECU side of the connector, I found the resistance was 50 Ohms to the solenoid. On the solenoid side of the connector it was 0 Ohms.
Meaning, the connector was the culprit.
I tried cleaning all the contacts but the resistance persisted. Finally hot wired across the connector. Resistance futile, sorry
couldn't resist that one....
Hot wire in place, 0 Ohms to solenoid, error messages cleared, off for a thrash and hey presto, back to normal and a smile back on my face again.
Worth the investment in a multimeter.
Got to thank Smaky though for the direction on this one.