Air Conditioning - Air Handler Fan

Okay, i am definitely a newbie and very ignorant mechanically. I have a 3 Ton Trane Unit outside that services the main floor of the house and a 2 ton unit that services the upstairs. They are both Trane an manufactured in 1996. Just yesterday, A/C repair technicians fixed leaks in the coil in large unit with some type of leak sealant that attaches itself to the leak by i am assuming some type of pressure. it is working fine now. however, the upstairs air handler i noticed the fan isn't coming on when i have the thermostat set to Auto. if i turn the fan to on mode, the fan works. or if i set the temp to like 6-7 degrees lower than displayed temp, it will come on. should i leave the thermostat on Cool mode PLUS fan to On mode?:confused:

Please specify which fan is not coming on. Is it the blower fan inside the house in the air handler or the fan outside on the condensor (assuming that this is a split system?)

Sounds like the stat is 6-7* out of calibration...