First, I'm just a casual user, so no definitive answers here. My experience has been with a T180 and an 863G, both of which have a similar control system to the S175, I believe. The seat belt light is not like cars these days, rather the computer just turns it on for 30 seconds (after start or after seat bar sensor activation, I'm unsure which) then turns the seat belt light off whether you have it fastened or not. There is no Bobcat Central hotline that I'm aware of, but wow, that would really be nice. This forum is as close as I've found. Now back to your problem: I thought the computer would sense the seat bar being down, the engine running, and the Push to Operate button, then issue the command to unlock solenoids (tilt and lift, plus traction lock) so the machine would operate. When the PTO button is pushed, the last light should come on and you frequently will hear a click as the traction lock solenoid is retracted (assuming the machine is not on an incline and putting pressure on the solenoid). That's the way I think it should operate. The fact that you're not getting the fourth light may indicate a computer fault since it obviously knows when you push the PTO button and when the seat bar is down. That really doesn't help solve your problem, though. Have you tried checking for computer fault codes by pressing and holding the "headlight" button for a couple of seconds? If no faults, it should display "none". Keep us posted on your progress. There are some very smart Bobcat techs on here who chime in on threads as they have time.

---Bobbie G