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They leak off vacuum when they fail.
I got curious and opened my old one too. The electrical bit functions just fine, I believe the diaphragm deteriorates in a way that it no longer seals. I tried cleaning and conditioning the diaphragm without luck. The solenoid functions by either applying vacuum to the wastegate actuator, or leaking off that vacuum. When mine failed, it leaked off vacuum continously, so the wastegate wouldn't close and I got no boost.
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