How to Troubleshoot Control Circuits on a Furnace Control Board

On a gas furnace, the furnace control board is essentially the heart of the system. If the control board is not working properly, or it is detecting a problem in the system, the furnace will not start up, leaving you without heat. Newer, high-efficiency furnace control boards are more complex than older models; however, all control boards serve the same basic purpose. When you turn the thermostat to call for heat, the control board first starts the induced draft motor, if your system uses one. The motor then causes a pressure switch to close. Once these steps have been completed safely, the control board ignites the pilot, starts the hot surface ignitor, or starts the direct spark ignitor, depending on your furnace model. The gas valve is then energized along with the blower to produce heat.

Instructions

- 1
  Locate the LED display on your furnace. Check to see if the green light is lit, indicating that you have power to the furnace and that the control board is operating.
- 2
  Examine the LED display for any trouble codes. When the control board senses a problem starting up the system, it will lock out the start-up process and begin to flash a trouble code on the LED display.
- 3
  Count the flashes on the LED display. Each furnace model will have a different set of codes; however, watch the display and count how many short and/or long flashes it displays. If the system is encountering more than one problem, it will flash one code, pause briefly and then flash the next code. Refer to your owner's manual for the corresponding problem associated with the code.
- 4
  Turn off the power to the furnace if you suspect that the control board itself is not functioning. Your control board is typically located in the blower compartment.
- 5
  Open the control board and check for a blown fuse. The fuse in your control board is similar to a fuse found in a car. When a fuse is blown, the wires inside the fuse are no longer connected. Replace the fuse if blown.
- 6
  Look for any wires that are touching the metal frame or grounding out if you encounter a blown fuse. If you find a damaged or loose wire, replace it

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