im throwing this one out there. hoping someone came across this issue, it has gas heat, with electric backup rtu. maybe 5 -7.5 tons. i was checking all the ac's at this new account (restaurant), came across this one that was off. the circuit board was about 6"x12" it gave a fault code of #4 which meant smoke alarm fault. in the unit it had a pan relay from the pannel com. and N.O. jumped it out reset the unit thinking the contacts in the switch were bad. same code. but once i take off the r wire going down to the tstat or a wire that is in the r circuit such as 1 wire off the smoke relay unit fires up and runs smooth.cools down to 72. so i was confused the book said if the unit cannot locate a thermostat it will run off the supply and return air sensors. i looked down the return duct and see a duct detector ima go back monday and try to reset it. but if its that it shouldnt run at all. it started pouring and i didnt get any info please don't ring my neck. but its something that failed or stuck open because as soon as i reset it the born lights up then flashes the code. it doesn't even start. im hoping someone by luck ran into the same issue.