How to Know if your Furnace Control Board Is Bad? (9-Step Guide)

Furnace acting up, and you think the control board could be the problem?

You’re in the right place!

In this proHVACinfo guide, you’ll learn:

  • Signs your furnace control board is bad
  • Supplies you’ll need to test your furnace control board (don’t start working without this!)
  • How to tell if your furnace control board is bad (9 steps – we make it easy!)

What are the Signs of a Bad Furnace Control Board?

Common warnings signs of problems with your furnace include:

  1. Warning lights on your furnace’s diagnostic system (see step 1 below)
  2. Temperature instability (especially when the thermostat is ok) and;
  3. Interruptions in the normal sequencing of your furnace

As its name suggests, the control board is the central hub for circulating power throughout your furnace.

This flat panel located inside your furnace takes the electricity coming from outside your furnace and sends it to the inducer motor, ignitor, gas valve, and other furnace components.

Supplies You’ll Need For Testing your Furnace Control Board

  1. Voltage Meter – The good news is that testing your control board isn’t actually that difficult. The bad news is that you do need your voltage meter; there’s no way to know for sure whether your control board is the problem without it. This handy tool can help troubleshoot not just your control board, but other parts of your furnace as well, so it’s worth making the investment in solid meter.
  1. Screwdriver – Some furnace access panels are held in place with screws. If that’s the case, you’ll need a good screwdriver before you can check your control board.
  1. Electrical tape – You’ll need something to hold down your furnace’s door switch while you test your control board; we recommend simple electrical tape for this.

How To Know if your Furnace Control Board is Bad (9 Steps)

Step 1: Check your furnace diagnostics

Most furnaces have a plastic or glass port with LED lights behind it in one of the access panels.

Those LED lights help you diagnose issues with your furnace by blinking in certain patterns (for example, it might blink five times, or blink three times with a pause).

The first thing to do is check that port and compare the light to the error codes found in your furnace manual or schematic (the schematic is usually found behind the lower access panel).

For example, certain error codes might indicate “low voltage”, which could further suggest your control board is bad.

Step 2: Remove the access panels

Most furnaces have two access panels which house the control panel, along with the other interior furnace components. These might be held in place with a latch, or could be held in with screws.

Remove both access panels, using your screwdriver if needed, before proceeding.

Note: In this guide, you’ll test whether electricity is flowing throughout your furnace correctly. For this reason, the power to the furnace will remain on throughout this guide. Always exercise caution when dealing with live wires.

Step 3: Tape down the door switch (if needed)

Most furnaces have a door switch that needs to be held down for electricity to pass through the furnace.

You’ll see this right when you take the access panels off, as the panels themselves typically hold the door switch down.

By default, the switch pops up when you take the access panels off, for safety. However, we need the electricity running in the furnace to test it, so use your electrical tape to keep your door switch held down while you work.

Step 4: Check the Control Board Indicator Light (if you have one)

The control board is generally located behind the lower access panel, in front of the blower. It should stick out right away as it has a number of wires running in and out of it.

The first step is to confirm power is getting to the control board.

This is very easy if your control board has an indicator light. The indicator light is an LED light that blinks when the control board is getting power.

If your control board has an indicator light, and you see it’s working appropriately, you’re in luck! Skip down to step 9.

Don’t seen an indicator light on your control board? No worries! With no indicator light, the best way to test your control board is to systematically check each juncture in the power flow.

Grab your voltage meter and move on to step 5!

Step 5: Locate the Common Wire on your Transformer

This step might sound complicated, but we’ll break it down for you!

The transformer is a small rectangular box attached to the furnace with screws.

It has four wires attached to it, two high voltage wires (usually black and white) and two low voltage wires.

One of those high voltage wires, usually the black wire, is the common wire. It will be labeled on the transformer (look for “COM” printed on the transformer).

Make a mental note of that common wire. It will come in handy in a moment!

Step 6: Test the Power Coming Into your Control Board

Next, locate the line voltage by locating the wire that runs from the door switch to the control board. It’s usually labeled “LINE” on the control board. This is the wire bringing 120 volts to your furnace.

Touch your meter leads to the metal connector of your line voltage and your common wire from step 5. You should see 120 volts on your voltage meter.

Step 7: Test the Power in your Transformer

In this step, we’ll make sure that the transformer (from step 5) is working appropriately. The purpose of the transformer is to “transform” (get it?) the 120 volts coming in from the high voltage wires down to 24 volts in the low voltage wires.

Touch your meter leads to each of the high voltage wires on the transformers. You should see 120 volts on your meter, so you know 120 volts is coming into the transformer.

Now, do it again with the low voltage wires. You should see 24 volts on your meter, so you know 24 volts are leaving the transformer.

If you see 120 volts coming into your transformer, but don’t see 24 volts coming out, your transformer is faulty and is likely what’s causing the problem.

Step 8: Test the Power in your Molex Plug

Trace each of those two low voltage wires back your control board, where they’ll connect to a Molex plug. Place your meter leads into the sockets of the Molex plug where the low voltage wires run, and you should see 24 volts again on your meter.

Step 9: Test the Power in your Terminal Strip

Last step! Locate the terminal strip in your furnace, usually at the edge of the control board. You’ll see 5 wires of different colors and corresponding letters printed on the terminal strip (R, W, Y, G, C).

The “R” wire is what lets you test for low voltage power coming out of the terminal strip.

Touch your meter leads to the “R” and the “C” wire. You should again see 24 volts.

And you’re done! If you successfully made it through all of the above steps and have power running from your line voltage through your transformer and Molex plug, out through your thermostat terminal strip, you know your furnace control board is working properly.

If at some point you stopped seeing the correct amount of voltage on your meter, that will tell you what exactly is causing the problem.

Stephen Marks

Stephen Marks

Stephen is an HVAC and home-repair enthusiast. He's here to answer any of your questions about HVAC!

Leave a Comment

Stephen Marks

Stephen Marks

Stephen is an HVAC and home-repair enthusiast. He's here to answer any of your questions about HVAC!

About proHVACinfo

You’ve got HVAC questions. We have the best answers from a network of seasoned pros.

Recently Published Guides