Why won’t shredder shut off on auto?

Generally shredders with an Auto setting use a photo sensor to detect when a sheet of paper is in the shredder. Chances are the sensor is being blocked by a piece of paper (an occupational hazard for a paper shredder) and most likely just needs to be cleaned.

Without knowing the manufacturer and model number of your shredder we can't give you any more specific information other than to just try cleaning out the input slot. See if you can locate the sensor, whether it's a mechanical switch or a photo eye and clear it out from any obstructions. That usually fixes the problem.