I have an older model lennox furnace and would like to know hwt the limit switch should be ste at to keep the blower from running too lon. The model number is G1203-110-6 it I removed the plate and itd set at 105 and 200 and the blower runs until the thermostat reads 78 and the stat is set at 75 and the stat is only 3 months old

The thermostat in the auto mode does not cause the furnace blower to operate. The control on the furnace you found controls the blower motor in the heat and auto fan mode. The 200 degree setting is probably a limit safety setting and should not be disturbed if it is so. The other setting could be moved a bit higher or lower to accommodate you. Some thermostats also have a setting for the heat anticipator and on many thermostats this is adjustable. Follow the directions that came with your thermostat to adjust the anticipator. You might be better off adjusting the thermostat anticipator first since it might have more to do with your problem than the fan and limit control.

Thank you I will try that when I get home. Ill look at my thermostat and see if there's an adjustment I can make