How Do I Know if My Furnace Control Board Is Bad?

What causes furnace control board failure?

There are a few different reasons that will cause a furnace control board to fail. Control boards have been known to last the entire lifetime of a furnace, but they do go bad.

Here are some common reasons why a furnace control board may fail. They’ll help in troubleshooting the issue as you diagnose what’s wrong and knowing whether repair is possible. 

  • Poor/Failed Solder Connections – This one might top the list of reasons for control board failure. The connections to the back of the control board and the soldering on the Molex plugs are known to crack over time. Of course, the soldering could have been poorly done at the factory, not making a strong bond and quickly coming loose. Either will cause the board to not function properly. Can it be repaired? Yes.
  • Thermal Expansion –Solder is a rigid metal and doesn’t flex. When there is warmth, there is expansion. This expansion will cause gaps between the stems. These expansion joints will cause problems either immediately or in the future. Constant expansion with warming and contraction with cooling can also make the soldering brittle and susceptible to cracking. Can it be repaired? Yes.
  • Issues with the Relays and Switches – Over time, the relays on the control board may start to burn and pit due to the arcing that happens on the contacts when the relays close. The high temperatures in the furnace can also melt the protective coatings on the relays. This will cause it to be misaligned and unable to function properly. Can it be repaired? Yes.
  • Stuck Switches – The switches on control boards are electro-mechanical switches. If the switch hasn’t been used over a long period of time it can become permanently stuck due to corrosion or dirt. You can sometimes tap on the relay to attempt to free the switch, but that just puts a band-aid on the problem. Try delicately cleaning it with Emery cloth for a potential permanent fix. Can it be repaired? Possibly.
  • Worn Switches – If there is an issue with voltage, either under, over, or a spike, this can cause the switch to wear out quickly.Can it be repaired? Possibly.
  • Transistor Issues – This is one of the first components that will fail on a control board. If there is a voltage spike or any static electricity, then the control board will go bad. This generally happens over time, but it is a common reason for control board failure.Can it be repaired? No.
  • A Power Surge – A lightning strike close to your home can cause a power surge throughout your home. It might not be evident that the lightning strike affected your home, but even if your lights and appliances are working properly, that single strike may have just completely fried your furnace’s control board. A brownout also can cause damage to your control board. The low flow of energy, followed by a normal flow of energy can also ruin the control board. Can it be repaired? No.
  • Static Electricity – If a technician happens to walk across a carpeted floor on their way to work on the furnace, they can build up voltage in their body. The first metal object that they touch will transfer the energy from there to the object. If they end up touching the control board that static electricity will damage the board and cause it to not function properly. This is called a “rookie mistake” for technicians, and ethical companies will replace your control board at their expense when this occurs. Can it be repaired? No.

How to Test a Furnace Control Board

If you suspect that your furnace control board is acting up and may need to be replaced or repaired, there are steps you can take to diagnose if the control board is malfunctioning.

First, there are a few necessary tools that are required to test your furnace control board. A voltage meter is needed. You will also need a screwdriver to remove the access panel and some electrical tape to tape down the furnace door switch – If this switch is Up, the furnace won’t operate, because the manufacturers deem it unsafe to run a furnace with the panel off. It’s a pop-up switch like the light in the fridge.

OK, here is how to test a furnace control board:

1. Check if there is a furnace error code – Many furnaces have a plastic or glass window/port that has an LED light or lights located within the window. The lights blink in a sequence to report an operational error. These are called “error codes,” and there should be a key located near the viewing port or in your owner’s manual that tells you what the light sequence means, that is, why your furnace isn’t working. An example of an error code that may signify that your control board is that the lights will indicate “low voltage.”

2. Remove the access panels to the control board –  There will be one or two access panels that will need to be removed. They will either be held in place by latches, tabs, or screws.

3. Tape the door switch – The switch will pop up when you remove the panel, and it will open/disrupt the flow of electricity. Tape it down to keep power flowing, so you can continue troubleshooting the furnace control board.

4. Check the indicator light (if available) on the control board – Most control boards have a LED light that will blink to signify that the control board is getting power. If the light is on, skip all the way down to step 9.

5. If the indicator light isn’t blinking, locate the common wire on your transformer – The transformer is a small rectangular box attached to the furnace. There will be four wires attached to it, two high voltage wires(most likely black or white) and two low voltage wires. The common wire will be labeled “COM” on the transformer, and it is usually the black high voltage wire.

6. Test to see if there is power coming into the control board – Locate the wire that is labeled “LINE” that will be connecting the door switch to the control board. Use your voltage meter, and touch the leads to the connector of the line voltage and the common wire. If everything is working correctly, you will see 120 volts on the voltage meter.

7. Test that there is power to the transformer – The function of the transformer is to change the voltage from 120 volts in the high voltage wire down to 24 volts in the low voltage wires. Using your voltage meter again, touch the leads on each high voltage wire. They should read 120 volts. Repeat the process with the low voltage wire, and you should see 24 volts for those wires.

8. See if there is power in the Molex plug – Follow the two low voltage wires to the control board. This is where the wires will connect to the Molex plug. Use your voltage meter and check to see if it reads 24 volts.

9. See if there is power in the terminal strip – The terminal strip is most often at the edge of the control board. There will be five wires of different colors and will have different letters to correspond with the wires (R,W,Y,G,C). The “R” wire tests the low voltage from the terminal strip. When you touch the “R” and “C” wires with your voltage meter it should read 24 volts.

At any point, if any of the expected voltages are not correct, you most likely need furnace control board repair or replacement. 

How Much Does It Cost to Replace a Furnace Control Board?

Average cost is about $265 for the part and another $100 to put it in.

The potential range is broader – the cost to replace a gas furnace control board varies greatly depending on the type of furnace you have, how old the furnace is, and where you live. An integrated furnace control board – one board controlling both ignition and function/sequencing of the furnace operations such as starting the draft motor, energizing the hot surface igniter, opening the gas valve, can run up to $500.

You can find gas furnace control boards easily on Amazon or other online retail platforms. When you factor in all the possible labor costs from a professional HVAC technician, you can expect to pay anywhere between $200 and $800 to replace your control board.

Can I Do It Myself?

Yes. DIY furnace control board replacement is possible. 

One of our Pick HVAC editors recently did it and reported that it’s a pretty easy job if you do one thing: Take a picture of the old control board with everything attached before you start pulling wire connections, so that you can reinstall the wires in the exact same way on the new board.

Pro Tip: It will take a furnace technician up to twice as long, at twice the cost or more, to figure out a mis-wired furnace control board replacement as it would to do it from the start. So, if you don’t trust your ability to exactly replace wire for wire, call a furnace guy. No harm in that.

Help – I bought a replacement control board and it doesn’t have the same wiring configuration!!!

Well, you might have the wrong board. Or, the board might be OK, but wire placement is different. It’s best at this point to get a furnace technician there troubleshooting the furnace control board issue.

This might help: Get free guidance and furnace board replacement cost estimates using the Free Local Quotes tab or toll-free number on this page.

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