Our control board replacement guide will help you to diagnose and replace (when necessary) your furnace control. There are basically two types of gas furnace control boards. The first type are simple ignition controls which control the gas valve, the ignitor (either a spark generator or hot surface ignitor), and a flame sensing circuit. These controls are more common on older less efficient units. The other type is usually referred to as an integrated furnace control. These controls perform the same functions as the ignition control but also control the furnace blower in both the air conditioning and heating modes as well as providing onboard diagnostics.
**NOTE** ONLY YOU CAN ASSESS YOUR ABILITY TO PERFORM THIS TASK. THIS GUIDE CANNOT COVER EVERY POSSIBLE SITUATION.
The integrated furnace control is the heart of the furnace. Most of them have the same basic sequence of operations that they control. It is critical to know what the control is supposed to do so that you can tell that it is operating properly.
On a call for heat, they first start the induced draft motor (if the furnace has one). Then, the inducer causes a pressure switch to close. The control then either ignites the pilot, starts the direct spark ignitor, or starts the warmup of the hot surface ignitor. After the pilot is lit and verified or the hot surface ignitor is warmed up (glowing bright yellow), the gas valve is energized. At the same time a timer begins to ensure that the burners were lit. In most cases a separate flame sensor proves that the burners are lit. After the burners are lit, a timer starts and energizes the furnace blower after the set delay. When the thermostat is satisfied, the control shuts off the gas. The inducer usually runs for about 30 seconds to purge the heat exchanger. A timer shuts the furnace fan off after a delay. This time delay is usually adjustable on the control board.
If the control does not cause the individual components to perform as described above, the control may be bad. Observe the operation and when the normal thing does not happen, you should check the operation of the related components. Most of the controls have built in diagnostic indicators. These are normally led lights that operate in flashes to indicate the problem. There should be a chart on the control or on the furnace access panel that explains the flashing codes. For many of the controls, there is a green light to let you know that the power is on and the control is functioning. If this light is not on, the control is usually reset by shutting the furnace power off for about 30 seconds and then turning it back on.
**HOT TIP** IF YOU HAVE TO RESET THE CONTROL, PERFORM MORE TROUBLESHOOTING TO FIND THE CAUSE. IT IS NOT NORMAL TO HAVE TO RESET THE CONTROL.
Once you have identified a defective control board, you continue your control board replacement by finding the proper replacement. This can be achieved by finding the manufacturer's part number or by using the furnace model and serial number. There are also universal controls that can be used for many furnaces.