Furnace Heat does not turn on? After starting the pilot light on gas furnace, I cannot carry heat...

After starting the pilot light on gas furnace, I cannot carry heat to start. When turning the lover switch from 'auto' to 'on', fan will start. Thought it be the thermostat, so replaced, and same situation. Fan will turn on, but when on 'auto', no heat.
Thought might be thermocouple, though I replaced second year.
Any thoughts?
Anybody next to first mitt experience next to...
Answers:    Check your transformer...240 in, 24 volts out.
We enjoy broken a terracotta tile on our...
Make sure the switch on the side of the furnace is not surrounded by the "off" position.