Write down the code on the LED display. Turn off the electrical supply and gas supply to the furnace before performing any work on it.
Check the air filter if the code indicates a problem with the airflow. If the air filter is dirty, rinse it under cold water and allow it to dry completely before putting it back into the unit. If your furnace uses a disposable air filter, replace it.
Check the supply air and return air grilles if the airflow is still obstructed. Ensure that the grilles are open and that nothing is blocking them.
Check the thermostat if the furnace fails to heat the building properly. Set the thermostat to “Heat” mode if necessary. Set the thermostat to a temperature above room temperature.
Locate the manual shut-off valve to the gas pipe. It needs to be open for the furnace to operate properly. The gas valve control switch should also be switched on.
Look for any possible obstructions around the vents if the furnace still fails to operate.
Press the reset switch if the light code indicates the furnace may have shut off because of high temperatures. This switch is on the burner box.