How to Troubleshoot a Condensing Gas Furnace

Check the power line connected to your furnace. There should be a metal box or panel with a power button on this line. Check to make sure the furnace is turned on. If it has been accidentally turned off, that is likely the reason your furnace isn't functioning. If this isn't the issue proceed to the next step.

Look into the inspection window on the front of the furnace and check the pilot light. If there is no flame, it is likely the pilot needs to be lit. Also, there may be a LED indicator light in the furnace. If there is a problem with the furnace the light may blink intermittently.

Lift the front top panel off of the furnace. Many of these come right off, but if yours is secured by screws, use a Phillips screwdriver to remove the screws from the panel.

Look on the back of the panel to see if there is a guide printed on a sticker. If there is, it will tell you the significance of the flashing lights. If there are no lights or guide, proceed to the next step.

Remove the bottom panel on the front of the furnace in order to access the air filter.

Lift the square-shaped air filter out of its compartment and inspect it. If there is dirt and grime caked on, it needs replacement. This is likely the reason your furnace isn't functioning properly.

Check near the motor housing for a button labeled "Reset." If there is a "Reset" button, press it and let the system reset. This could solve your furnace problems.