I drive a Crown PE-4000 stand-on forklift. It seems to operate just fine most of the time, but the wrench light flashes and "Code 21" flashes on the screen, and occasionally without warning, the forklift will stop travelling, whether it's in the middle of an aisle, during travel, or attempting to resume travel. I will have to reset/restart to get it to work again.

The warehouse mechanics have looked at it a few times but apparently they haven't found what's causing the issue, because the problem still happens.