My combustion air fan will kick on and burner will lite but fan will not kick on. When I put the termostat in cool the blower will kick on but won't in the heat setting. It doesn't matter if it is in auto or on position. Won't kick on in either

Is this a new stat or have you touched the wiring to the stat or furnace.

Put a new thermostat in. Replaced a sensor and cleaned the other.