Last Updated on May 18, 2023 by Charlotte HVAC Specialist
You turn on your heat, then smell burning plastic or metal, or a strange electric smell. What is it? It’s from the furnace. It could be the control board. Does my furnace have a control board?
Furnace control boards can go bad. They can short-circuit and fry, giving off that odor. Is there any hope this is a DIY job? Here’s how to check and know if it’s the control board. It might be replaceable. Here’s what to know and the steps to take.
How to Check a Control Board
Don’t panic. You don’t need to trash the whole furnace and heating system. First, let’s figure out if it’s the control board or some other issue.
- Take off the furnace access panels – Remove the front access panels. The control board is usually located in the bottom section, near the blower.
- Check the control board – Now you can see the control panel. Does it have any burn, melting, or blackened marks like a spark or short happened? Are there strange smells? Sometimes a control board will have clearly exploded. If this is what you see, the control board is done.
- Check the control board light – The next thing to inspect is the board’s diagnostic or indicator light. Normally, the light will blink a particular number of times. It’s meant to tell you what is going on in the control board. For example, three flashes can indicate the furnace’s inducer pressure switch is not working. If the board has no power, the light will be off. Inspecting the control panel’s diagnostic light will tell you exactly what is happening with the furnace. In fact, many times there won’t be any concern with the control board itself. The issue is in another part of the furnace.
- Test power going to the board – If the control board’s light is not blinking, you should check if power is getting to the control board. Check the wire that powers the control board. The wire is generally black with “120VAC Hot” on it. If the furnace has a door switch, you should see it is pressed in. You can use tape to keep the switch down. You’ll need a multimeter to test for electricity in a furnace. If the control board has 120 volts of power entering it, then it has power. If the board has power, but the diagnostic light is still out, the board is most likely failed and needs to be changed.
- Check the terminals – After you know there is power to the control board, you can move on to test other parts of the furnace. Before anything else, turn off the furnace. Next, using a wire, connect the R and G terminals. Turn the furnace on. The blower should start. To test the heat, connect the R and W terminals. To test the cooling, connect the R and Y terminals. If the furnace turns on after testing the terminals, there isn’t a problem with the control board. The problem could be with the thermostat.
- Check the power going out from the control board – The next thing to check is for power exiting the board. This is tough since you have to know what each terminal on the panel is for. First, use a multimeter to get the output from the control board. Most control boards use relays to activate and turn off other components such as the blower, ignitor, and valve. The majority of heating systems have a wiring diagram on the inside of the door or panel. The diagram helps you locate what points you need to evaluate. You’ll also need to understand the furnace’s order of operations. Occasionally, a control board won’t activate your furnace but it’s actually another issue such as a pressure switch. If you can’t find other issues with the furnace, and there is no voltage output to the rest of the components, then it’s possible the control board is done.
What to Do if the Control Board is Shot
If it looks like the control board is broken, you’ll almost always have to replace it. Replacing a control board is complicated because many wires are attached to it. You must have the exact, compatible control board for the furnace.
When replacing a control board, it’s a valuable idea to take a picture of the existing board to see how it’s wired. You can refer to the photo when installing the new one.
Changing a Furnace Control Board
Changing a furnace’s control board is a complex task. It’s recommended that, if you’re in doubt, contact a professional technician. If you have the knowledge and what to give it a DIY try, here are the main steps.
- Shut off power to the furnace – Cutting the power to the furnace is critical before working on it. There are wires and circuits. You don’t want a hazard or injury.
- Open the furnace – Take off the access panels on the furnace and get to the control board. On some models, you might need to move or remove other components to get to the control board. This is when you should take photos of how it’s set before removing it.
- Disconnect wires from the board – Remove all of the connectors and wires until nothing is attached to the board.
- Take the control board out of the furnace – After disconnecting everything, take the board out. The control board is usually attached by a couple of screws.
- Install the new control board – Here is a good time to set any kind of switches or jumpers if the control board has them. Follow the exact same settings the old control board had. Attach the new board inside the furnace the same way the old one was.
- Reconnect wires – Now reconnect the wires and connections, using the pictures of the old board, to the new board.
- Close the furnace – If you have to move other parts to get to the control board, reassemble those. Then put the access panels back on.
- Turn the power back on and test the furnace – It’s the moment of truth. Turn the power on. Turn the furnace on. Test it by changing the thermostat.
At South End Heating and Air, we specialize in HVAC and furnace repair, call us for a free consult. We’ll evaluate your system and help make recommendations for optimum value. After all, we want to keep you cool all summer long and warm in the winter. Just schedule a visit with one of our technicians to talk about how we can help with your heating needs. Would you like to learn more options our techs can help you with? Give us a call 704-684-5339. We also can address your plumbing needs.