casual noob biker here, bought this bike a couple days ago and after 20 miles the left crank arm and pedal is stripped, the pedal just fell out, the shimano crank arm taking the bulk of the damage (completely stripped).
wondering what my options might be from here, I don't want a lot of down time on this bike so waiting for a tedious return doesn't sound appealing.
Anyone recommend the best solution, or what crank arms I should get as a replacement? Or if I should go the warranty route, not sure if my issue would be with diamondback or shimano?
It was almost certainly an assembly oversight and not a manufacturing defect at the heart of the problem. If you bought the bike locally, you should be able to get this resolved pretty easily by bringing the bike back to the seller for replacement/repair.
In a worst case scenario, you could replace the crank arm yourself. A whole Shimano FCM131 crankset (the model on your bike) can be found in the $20-30 range online. You'd also need the appropriate crank puller to remove the crank arm. If the Park Tool CCP-22C crank puller would work for that crankset, you should be able to pick one up for $15-20.