ZD's have a seat safety switch(*** must be in Seat to start), brake Safety Switch(Brake Must Be On to Start), PTO Safety Switch(PTO must be disconnected to start) and steering levers have a safety switch on each one(Levers Must be "open" meaning the levers opened so you can enter and exit the seat) The ZD safety switch's are small micro switchs, some hard to get at,there are a total of five. To test, using your finger work each switch and listen for the "click" it must be in a quite area, because the "click" isn't very strong. If you find one or more that doesn't click, replace it. If they all have a click when working the small arm/lever then you must disconnect and test each with an OHM Meter. If replacing a switch, make sure you order the same switch your replacing, there are "NO" and "NC" switch's. One has a Black connector at the wire's ends, the other has a Gray connector. Hope this helps. Ken