Updated: Oct 25, 2022
Diagnosing if a control board is bad on a furnace, air conditioner, or heat pump may seem difficult but here are some simple tips and processes to make the job a little easier.
It is difficult, if not impossible to know if a control board is bad if you don’t know how the system it is attached to works. For a furnace, you need to know the sequence of operation for the heat to turn on. (Check out our video on that.) For an air handler, you will need to determine if the unit is connected to a heat pump or A/C and if the unit is equipped with electric resistance heat strips. If the unit does have electric strip heating, is it the sole source of heat or a backup source of heat. If there is a problem with the outdoor unit, view the type of defrost board and other system components.
Next, when approaching the HVAC system, take a few minutes to get your head wrapped around what you are working with. Don’t be ashamed of taking some time to understand how the system works by looking at the thermostat, outdoor unit, indoor unit, and wiring diagrams on the unit shrouds. You may know how a system generally works but it is good to visualize the various parts inside because this can change from unit to unit. Remember that the control board needs to control these various components. View the components inside and this will help you figure out what the control board is supposed to do.
Next, determine what type of control board is inside the unit. Depending on the system, the control board may only control certain aspects of the system and not others. For instance, earlier furnaces may be equipped with only an ignition control board and not a fan control board. A board that controls both ignition and the fan is called an integrated furnace control board (See this video on an IFC board). An air handler board may be a simple fan delay board to control a PSC (Permanent Split Capacitor). In other cases, the control board is made to control an ECM (Electronically Commutated Motor) multispeed blower motor. Other boards may control an ECM variable speed blower. By noticing the type of blower motor, this will help you determine the function of the control board. Check out this video on determining the type of blower motor.
Every time you are on a service call, take time to look at and absorb the wiring diagrams. The Legend, shows what the abbreviations stand for, the Connection Diagram shows the color wires and where they connect to, and the Schematic Diagram shows the electrical path to determine how the system works.
The real secret to control board troubleshooting is that there is no official test to prove that the control board is bad. It is more about knowing if the control board is controlling components as it should. When there is a 24 signal on the W terminal of a furnace control board, the board should allow power to the inducer motor. When there is a 24 volt signal on the G terminal of a board, the board should allow power to the fan motor. If there is no output to the motor in these circumstances, then you know that the board is at fault. However, there could be a problem with a safety switch being open. This will usually stop power to the ignitor or heat strips but the safety switch usually does not stop power to the fan. In fact, often times the safety device will allow the fan to be powered all the time in a furnace. In this case, test the safety limits to see if one is open. The control board gets blamed way too often as being the problem when actually, there is a problem with another component.
If you are suspecting that the board is bad, make sure you know how the system is supposed to work. Look at the LED status codes (if the board is equipped). This may lead you to a problem with a component and not the board. Look and smell for a burnt component on the board. Check for loose wire connections on the inputs or outputs from the board. I have often found loose wire connections leading to a problem. These have been on the thermostat wire connections, 16 pin connectors for variable speed blower motors, high current connection points for motors, as well as loose or cracked solder joints on the board. Sometimes the problem is found by wiggling the wire connector or the wire itself inside the connector at the board. Make sure to be safe and protect yourself from electrical danger when current is being drawn. Make sure to wear rubber gloves and treat the system with caution and respect.
Lastly, you could have a relay on the board that is intermittently connecting. The relay connection points may be bad due to high current being drawn across them. The connection points of the relay may be burnt and pitted. Relays on a board are noticed by a high square box. Under the shell of the box is a normal electrical relay with a coil and contacts. Solder joints can be fixed in the field but a bad relay on a board causes the board to be bad and replacement as the only quick option to get the system running. These relays are usually soldered into the board and each board may have a different sized relay. It is usually quicker to locate and purchase a new control board than to locate and solder in a new relay on the board.
Keep a cool head when troubleshooting systems and always remember that to prove a control board is bad, you must prove that the other components are good and that outside factors such as airflow or condensate clogs are not the problem!
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Published: 9/23/2020 Author: Craig Migliaccio
About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 15 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel & https://www.facebook.com/acservicetech/