Furnace’s Control Board is Bad? Here’s What to Check

Do you smell that? Is it the smell of burnt plastic or electronics? Then it’s probably your furnace’s control board that went bad.

Control boards can go bad for many reasons. A lot of times, a short circuit in the board will cause it to fry.

Other times, an electrical component will malfunction and cause strange behavior. Or if your furnace won’t turn on at all.

In this article, I’ll discuss what to check if your furnace’s control board is bad. I’ll also tell you how to replace a control board in your furnace.

How to check if your furnace’s control board is bad

Before you decide to condemn your control board, you need to first look at a few things. A lot of times the control board isn’t the thing that causes issues in a furnace. That’s why it’s important to figure out if your control board is even bad in the first place.

Here’s what you need to do:

Remove the furnace’s access panels

Take off the front access panels to your furnace. The control board is usually located in the bottom access, near the blower.

Inspect the control board

Take a look at the control board. Does it have any burn marks? Are there any weird smells coming from it?

Sometimes a control board will have obviously blown up. In that case, your control board is definitely bad.

Check the control board diagnostic light

The next thing that you should check is the control board’s diagnostic/indicator light.

Usually, the diagnostic light will blink a certain number of times, depending on what is going on in the control board. For example, 3 flashes could mean that the furnace’s inducer pressure switch is not triggering.

Or if your control board has no power, then the light won’t be on at all. Checking the control board’s diagnostic light will tell you exactly what is going on with the furnace.

In fact, most times there won’t be any issue with the control board itself, and the issue is coming from one of the furnace’s other components.

Check the control board fuse

If your circuit board loses power, it could be due to a blown fuse.

The fuse that’s in most furnace circuit boards is a 3-amp blade fuse. These are the same types of fuses that you can get at any auto parts store.

The easiest way to tell if your circuit board’s fuse is blown is to look at the center area of the fuse. If it looks like there is a black puff of smoke in the fuse, then the fuse is blown.

If your circuit board’s fuse is blown, it’s usually due to one of two things:

1. A short circuit somewhere in your thermostat wiring
2. A bad component on the circuit board (such as a relay)

Test power going into the control board

If the control board’s diagnostic light is not blinking, then you need to check for power going into the control board.

Find the wire that powers up the control board. The wire is usually black and labeled as 120VAC Hot. If your furnace has a door switch, you need to make sure that it is pressed in. Use some tape to keep the switch pressed down.

If your control board has 120 volts AC going into it, then it has power. If your control board has power, but its diagnostic light is not on, then the control board is likely bad and needs to be replaced. NOTE: You’ll need a multimeter to test for power in your furnace. 

Test the control board transformer

Some control boards require a separate 24-volt power source from an external transformer. In some cases the transformer can go bad, causing the control board to turn off from lack of power.

If your furnace has a transformer, you can check the output voltage of the transformer using a multimeter.

Use the AC voltage setting on your multimeter to measure the voltage on the 24-volt side of the transformer:

• If you measure around 24-28 volts coming out of the transformer, then the transformer is okay.
• If the voltage coming out of the transformer is way below 24 volts, or the transformer is making an excessive buzzing noise, then the transformer is bad.

Jump the thermostat terminals

After you confirm that there is power going into the control board, you’re going to do some tests.

First, turn off your furnace. Next, using a wire, connect the ‘R’ and ‘G’ terminals. Turn your furnace back on– the blower should come on.

To test the heating, connect the ‘R’ and ‘W’ terminals instead (see pictures below). To test the cooling, jump the ‘R’ and ‘Y’ terminals.

If your furnace turns on after jumping its terminals, then there is no issue with your furnace control board. The problem is likely the thermostat instead.

Test the power coming out of the control board

The next thing to check is if there is power coming out of your control board. This is tricky because you need to know what each terminal on the control board is for.

Begin by using your multimeter to test the outputs from the control board. Most control boards use relays to switch on and off different parts such as the blower, ignitor, and gas valve.

Most furnaces have a wiring diagram printed on the inside of their door or panel. The wiring diagram is useful for figuring out what points you need to test.

You’ll also need to know the furnace’s sequence of operations. Sometimes, a control board won’t turn on your furnace because of some other issue, like a pressure or high limit switch.

If there are no other issues with your furnace, and there is no voltage output to the furnace components, then it’s likely that your control board is bad. 

What to do if your furnace control board is bad

Unfortunately, if you have a bad control board, you cannot do much besides replace it. While it may be true that you can solder on a replacement component, it’s not worth it most of the time. 

Replacing a control board can be a complex task since there are many wires connected to it.

Also, you’ll need to make sure that you get the right replacement control board.

When replacing a control board, I usually take a picture of the existing board, that way I can see how it was wired. Then I can refer back to the picture if I run into any issues when installing the new board.

As with everything else, if you’re unsure about working on your furnace’s control board yourself, it’s probably best to leave it to an HVAC professional.

How to replace a furnace control board

Replacing a furnace’s control board is a fairly complex task that should only be done by those with sufficient know-how.

If you’re still dead-set on swapping out your furnace’s control board yourself, follow these steps below:

1. Turn off the power to your furnace. Shutting off the power to your furnace is a critical step since you will be working with wires and electrical circuits inside the furnace. You don’t want to get shocked!
   
   
2. Open up the furnace. The next thing to do is to remove the access panels from your furnace so you can get to the control board. Depending on the model of the furnace, you may have to remove other parts to access the control board.
   
   Once you get a clear view of the control board, take a few pictures of the board and its wirings. The pictures will come in handy later when you’re trying to reconnect everything.
   
   
3. Disconnect wires from the control board. The control board will have all kinds of wires and connectors attached to it. Remove all of them so that nothing else is connected to the board. 
   
   
4. Remove the control board from the furnace. After you disconnect all the wires, remove the control board from the furnace. The control board is usually held in by a few screws.
   
   
5. Install replacement control board into the furnace. Now it’s time to install the new control board.
   
   At this point, it’s a good time to set any dip switches or jumpers if your control board has them. Just follow the same switch/jumper settings that your old control board has.
   
   To install the new control board, screw it down inside the furnace the same way that the old one was.
   
   
6. Reconnect wires to the new control board. Now it’s time to make all the connections. Use your pictures of the old control board as a reference (you remembered to take pictures right?). Reconnect all of the wires and connectors into the new control board.
   
   
7. Close up the furnace. Now it’s time to put everything back together! If you had to disassemble anything extra to get to your control board, don’t forget to put those pieces back too. Close up your furnace by putting all of its access panels back.
   
   
8. Turn the power back on and test. Finally, the moment of truth. Turn the power back on to your furnace and test its operation at the thermostat. 

If everything is working again- great! 

If your furnace is still not working, it could be due to one of the following reasons:

• There is another issue with the furnace (limit switch, flame sensor, etc.)
• The thermostat is not working
• The wiring on the control board is wrong

In any of the above cases, you’ll need to do some further troubleshooting. It might be that your furnace’s control board wasn’t even bad in the first place. That’s why it’s important to check your furnace’s control board first.