Thermostat Not Working in "AUTO" mode. Everything works fine in "ON" position. Have replaced the main power switch and the Thermostat... but its still not working.

It is a gas heater/

What do I need to check next??


Thanks for any assistance.

BoneDaddy2004

Is the auto mode for fan operation or to change "automatically " from heat to cool?


What type of furnace is this? i.e. 80% or 90%, more details the better to get a good idea what we're working with.

With the fan switch turned to auto and the mode switch to heat turn up the thermostat all the way. Did the burners light up? Did you here any sounds at the furnace? Have someone turn it up while you stand near the furnace. What brand and how old? That information always helps, and if you know the efficiency.

The low speed blower relay. Most furnaces have 2 speed blowers controlled by 2 different relays. On actuates the high speed one keeping the limit switch from shutting the furnace off. Try a rap to the control board about 2 minutes after the gas comes on. If the blower takes off, the relay is going bad.