Why does my iPod Touchless trash can slam shut?

I have an iTouchless 13 Gallon Stainless Steel Sensor Trash Can (DZT13P). The owners manual is at this link. https://files.bbystatic.com/UJimZUXqRBH%... The can opens and shuts fine but has started to slam shut loudly the last couple of days. I’ve tried replacing batteries, running with the corded power, cleaning the sensor, cleaning the hinges, … but it still slams shut. I disassembled as much as I can and I don’t see anything wrong: the spring looks okay, nothing obviously wrong with soldered connections, the circuit board looks okay, …. I was not able to see all of the teeth on the plastic gears without more tedious disassembly. Does anyone have a suggestion? Any advice will be greatly appreciated. Thanks in advance.