The shield opens when it's supposed to, but closes very quickly after (1 sec). When I run diagnostics, the shield hold doesn't do anything unless I pull the shield up all the way - then the shield will hold a bit. Shield power seems to work fine - the solenoid has enough power to shoot the shield up to full height. I disassembled the shield/solenoid but everything appears ok. Diode and wires are all snugly attached, spring on the solenoid looks good. There's a switch on the mechanism but nothing ever touches it so I'm not sure how that's supposed to work.

Pictures of the mech are attached. Any ideas of how to troubleshoot are greatly appreciated.

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Awesome, thanks so much to you both!

Bumping this back up for more help.

I have replaced the switch and replaced the diode (twice, just in case I didn't do it right the first time) but the shield still doesn't stay up for more than 2 seconds after initially going up. Shield hold in diagnostics doesn't do anything. Any more ideas? This is the only thing that is still not working correctly on my machine and I would love to get the game to 100%. I've read every thread on pinside and every mention of the shield through google but I can't find any other ideas.

Thanks in advance for any help.

When you choose shield hold in diagnostics, does the shield go up and stay up? Mine just stays down so I’m thinking this is not a switch problem (but I don’t really know what I’m doing so I could be totally wrong. I’ll have to keep fiddling around with it I guess. Just very frustrating.

I don't know how to interpret the switch test screen. I see a number of green squares, some white with O in the middle (these are the ball trough optos I think). I don't understand what any of the options are in the switch diagnostics screen when I press the start button either.
I've tried putting the playfield up and testing the shield hold diagnostic while I manually press the shield switch but it doesn't make any difference - the solenoid weakly tries to raise the bar but it never goes all the way to the top and it never holds. Regular shield power is strong - the shield shoots up to the top with force.

Finally fixed the issue and figured out how the switch test works. For those who are new to the game (like me), the switch test works by manually pressing any of the switches in the game - the diagnostic will beep if the switch is registering correctly. In my game, no switches were reporting errors. However, even though the switch was working correctly, it was not getting fully triggered by the shield. I had re-installed the switch at least a dozen times trying to get it just right, but on the thirteenth try I finally succeeded. The shield hold is now working as intended. Hopefully this helps another new owner to the game. Thanks to everyone for taking a look and providing suggestions.