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Here is my write up on a successful repair of my D90. TL;DR it's the "aperture motor gear". There is a small gear buried deep in the camera that has cracked and will not drive the aperture/shutter. https://www.reddit.com/r/Nikon/comments/1rsyzgw/d90_apature_motor_repair_photos/ The parts cost like 10 bucks: https://www.amazon.com/Haoyull-Autofocus-Aperture-Durability-Material/dp/B0DCMFPRM5 However it's a very challenging repair. I mostly used this video for the steps: https://www.youtube.com/watch?v=JPMkNjES2pk It is very difficult to do, it takes serious patience, de-soldering/soldering, the ability to discharge the 300V flash capacitors safely. However it can be done. This is a common failure mode of the Nikon D-series cameras. You might be better off just buying a used D90 camera body unless you're set on keeping that one. It's also very easy to damage other components inside the camera when carrying out the repair. Good luck!