I can turn on the controller, but after two seconds it immediately turns off. I plugged in with a cable meter, and the...

After about 8 years using the Steam Controller, the right bumper stopped working. It no longer clicks audibly when...

So this is pretty incredible. After years of using the Steam Controller regularly, the two top pieces of the back cover...

I found the 3D print file of the replacement part, gave it a try with Polycarbonate from PCBWay's 3D Print and Delivery service, and got it installed in my controller, working well so far. I made a guide on the procedure here.

According to the teardown, the Steam Controller is relatively easy to take apart with only a T6 screwdriver. You would need to reach Step 5 to get to the thumbstick (carefully unplug the ribbon cables on the trackpads before removing from the chassis). That should at least show if there are any physical obstructions causing the behavior you observed. Brush around the stick with a cotton swab dabbed with some isoprophyl alcohol. Worst case scenario, the thumbstick is soldered to the motherboard, so it would need to be desoldered to replace it. I'm personally not sure which would be a suitable replacement module for Steam Controller.

I did my own teardown, and while I can't confirm if the analog sticks are similar to the ALPS brand in an Xbox controller, they're definitely not Hall Effect. They're green potentiometers and friction-less mechanical sticks. Interestingly, the sticks are part of their own board and can be isolated from the main mother board, so a mod may be possible with some probing. About the Luna's analog sticks