I have a Carrier furnace with an automatic ignition. The fan is constantly running but it is not heating. I had this problem before and a tech came out and reset it but I cannot remember what he did. I cleaned the permanent filter and replaced it and I am getting a LED code 33 which states "Limit or Flame rollout switch is open" this code will then change to LED code 13 "Limit or Flame rollout switch lockout". It will then change back to code 33 after 3 hours. I already checked the breaker and reset that but the thermostat is not displaying anything. What do I do.

If the gas comes on and quickly goes off, the sensor, Limit or Flame rollout switch, that tells the controls the gas lit is not working. It will be around the heat exchanger and have 2 wires running to the controls. Make sure it is clean and properly connected. If the furnace still won't work, replace it.

If the gas never comes on at all, check the gas valve for 24 volts across its terminals. If not, make sure usually a blue wire connects to one side of the transformer, and usually a white wire connects to the W terminal of the controls. The white wire in turn should be connected to the red wire at the thermostat.