If your fan will not stop, it could be caused by several things. It could be the contacts inside of the thermostat sticking. It could be caused by the fan mode switch in on versus auto. It could be a sticking contact on the fan control card in air handler or a stuck contact in the fan control head if it is a X13 or ECM motor.
First, easiest way to see if stuck contact on the thermostat, gently tap the thermostat, this sometimes adgitates the contact and allows it to open. If this works you need a new thermostat.
Next problem is checked on the themostat fan mode switch, it should be in auto. I have had several customers who inadvertantly pressed the fan mode switch and did not know they had done it.
If the fan is still running, the problem is in the air handler fan control card. There is high voltage in and near the fan control card and I would recommend getting a professional to check it out. The voltages in most thermostat is about 24 volts and even though it can tingle you it is not generally harmful. The high voltage you get into the air handler can either be 240 volts and in commercial unit it can be 440 volts.
Hope this helps.
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