The control circuit of a furnace manages a variety of data produced by the thermostat, the sensors near the burners, the ignition system and the blower mechanism. It is in charge of starting up components in the correct order and then shutting them down properly when the furnace finishes its job. A control circuit can fail in several key ways, requiring require a replacement of the board or nearby components to keep the furnace operating correctly.

One sign that the circuit control board has failed is a lack of any type of response. For example, the thermostat is sending signals to the circuit board, but wires have lost the connection or the panel itself had burned out. If you have an ohmmeter, you can test the wiring running to and from the circuit control board to see if the wiring is at fault or if the problem lies within the circuitry itself. Replacing a few wires can be less expensive than installing an entirely new panel.

Circuit boards also includes a timer that controls the furnace blower after the burner or element has been lit. The timer itself can fail and lead to a properly operating burner but the blower fails to turn on. The relay switches and pressure switches that the furnace uses to open and close separate circuits can also fail, causing the timer to work but never send signals elsewhere. When the internal timer fails, you may need to replace the entire control board to fix the problem.

Thermistors and thermocouples send signals to the control board indicating the heat levels of the pilot light and burners. This helps the control board switch on its timer, shut down the furnace if temperatures are excessive and start the primary burner. These sensors can become loose or may have failed.

Consulting an HVAC professional to take a look at your furnace may be the best alternative and not save you the effort but also get the job completed quickly and safely. Modern furnaces have diagnostic indicators that show a series of lights. The pattern indicates particular problems. Charts are built into control panels or are available in manuals for more detailed looks. You may try to reset your furnace entirely to see if a diagnostic error is preventing the system from working.

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