Furnace will not ignite when thermostat is turned on and fan will not shut off . Code 33 comes on over and over . Using an ohm meter tested limit switch for what apears to be the flame rollout . Rings closed . However limit switch that apears to be sensing a vaacum or pressure of exaust is open. Also exaust fan is off during this . Note only furnace fan stays on and does not shut off regardless of thermostat condition