Yesterday my furnace stopped heating my home. It is 3 years old & I did some minor troubleshooting. The filter is not clogged and the blower is circulating air normally. I checked inside the unit & the ignitor coil is heating and starting the gas jets burning, but they shut off after about 10 seconds of burning, so no heat is entering the blower and I just get cold air. Since these functions seem normal, is it possibly a problem with the thermostat not telling the heater that it needs warm air? No matter how high I turn the temperature up on my wall thermostat, the furnace will not turn on. It only turns on when I change the setting from 'auto' to 'on'.

It isn't the thermostat. It is starting the heating sequence. More like the flame detector, a little ceramic dodad with wires right where the flame will hit it. If it doesn't send a signal to the board, it will shut the gas off in a few seconds. Clean up the metal contacts. Replace it if broken or cracked.

Look for the flame sensor. Clean the rod with emery cloth...

http://www.pse-usa.com/pics/sensor-1.jpg

This is on point... I believe that this might me the most common problem.