Furnace Troubleshooting Guide

Most modern furnaces have a control board with diagnostic lights. This control is commonly located either in the furnace blower compartment or in the furnace combustion compartment. BEFORE you remove furnace doors check to see if there is a viewport on the blower door. If so look through to view the furnace control.

Is there and LED illuminated or flashing a code?

If there is a LED light or a code flashing, record this information before opening the furnace doors. When the blower door is opened power to the furnace controls will be automatically shut off.

If the furnace does not attempt to operate:

Check thermostat for proper temperature. The thermostat must be set above existing room temperature to initiate furnace operation.

Check to ensure that furnace is in HEAT or AUTO mode.

If there is NOT and LED light illuminated, ensure that power is on to the furnace. Check the furnace emergency switch is on and check any external furnace fuses.

If the furnace attempts to operate but fails to start properly:

This means that the furnace has power, and is getting a heat signal from the thermostat, but is not lighting properly. Most commonly the furnace will attempt ignition several times and then lock-out.

Check gas supply. Check that the manual gas valve shut-off in the gas pipe leading to the furnace is OPEN.


Check the control switch on the gas valve. Is it in the ON position?

Check for manual reset switches in the burner compartment. Attempt to reset manual roll-out or limit switches and attempt to re-start.

Check the condition of the vent. If the furnace has a traditional chimney vent check to ensure the condition of the vent. If the furnace vent is plastic and terminates horizontally, check that the vent pipe is properly supported and that there are no obstructions visible at the exterior vent termination like snow or ice, debris, or insect nests.

If the furnace operates but it cycles on and off or heats the space more poorly than when it was last serviced or installed:

Check the air filter, and replace or clean if dirty.

Check to ensure there are no obstructions to furnace airflow. Make sure that return registers are not obstructed, and that most supply registers are open.