These truck have chain slack and chain broken switches. They are under the seat. I used to see these go bad every once in a while. Replace the switches. They are cheap. The chain broken switches activate when the chain has more slack than the chain slack switches. It as been a while since I worked on these. It think there are four switches total under the seat. I think the switches wear because the operator sets his forks on something and activates the chain slack switches frequently.