I'll begin by saying I feel sorry for the little buggers that get caught in these things but it does seem to be more humane (not to be debated) than mechanical traps.
I have two rat Zappers and between them have zapped over 85 mice and 10 rats. It's hard to believe this thing will retire a large Norwegian roof rat which is as big as my foot, shoe and all, but it does. We recently had a new infestation in my garage so I dug out the Zappers and found neither to be working. You can test them (using fresh batteries) by using a long screwdriver inserted in the back of the zapper grounding the back plate to the center insulated island. Once grounded, lifting the screwdriver off the insulated plate should draw an arc up to 3/8ths of an inch long. The first I repaired was only drawing about 1/8th of an inch. After soaking it in soapy water for an hour and scrubbing, I was able to get the proper 3/8ths arc with a healthy buzz to go with it.
The second I worked on would hardly spark and after cleaning it with soapy water, the problem remained. Opening it up, I found the underside of the circuit board to have some mild corrosion so I cleaned it with IPA and a toothbrush. I then noticed that the crimp on one of the larger leads running to the metal frame looked bad so I took the time to solder it up. I also found what looked to be fine wire on the circuit board like from a wire clipping. Soldering, cleaning and removing the fine wires corrected the problem. This one was then able to generate and hold a healthy spark. It almost looks like you could weld with it.
Anyway, I expect the rodent count to climb. It's amazing how much damage rodents will do to soft plastic wires, etc. I don't understand why they like some expensive wires more than others. By the way, peanut butter on a small square of paper towel works the best for bait. If I can clean the trap in time, I throw the carcass in with the kids king snake. Much cheaper than 3.75 for a frozen adult mouse.
Jerry