3 Signs of a Bad Furnace Control Board

Think your furnace’s control board has gone bad?

Signs your furnace control board is on its way out are unstable temperatures, the furnace is acting strange and the indicator light is on.

Below you’ll find out how to diagnose these issues, where the control board is, and how much it costs to replace.

Let’s get started.

Signs Your Furnace’s Control Board is Bad

Unstable temperatures

A bad furnace control board can cause your furnace to run too long or shut off before the desired temperature is met.

The control board is responsible for turning off components of your furnace, and if the control board is going bad, then your furnace will not know when to turn off and on.

Before you conclude it’s your control board, check the thermostat settings.

If the thermostat is set properly, then your control board being bad can cause unstable temperatures in your home.

Furnace acting strange

If your furnace control board is bad, then your furnace may start operating strangely.

If the blower motor stops working suddenly or continues to work when the furnace is off, or your gas jets don’t activate or the ignition doesn’t come on at the right time, then your control board is causing these malfunctions.

Indicator light is on

Your furnace control board has an indicator light that indicates if it’s malfunctioning.

If your furnace isn’t working properly, check the control board inside the access panel to identify any red lights on the control board.

Your furnace should have a diagnostic chart near the control board that will decode the indicator light.

Not all furnace control boards will have an indicator light, so don’t worry if you don’t see one.

If your control board has power but the indicator light isn’t on, then you’ll need to replace your control board.

To confirm your control board is out, you’ll have to test it with a voltage meter.

What Does a Furnace’s Control Board Do?

Consider your furnace’s control board as the brain of the operation. The control board operates the ignition, flame sensors, blower motor as well as gas valve.

Your furnace is a complex machine, and without the control board letting your furnace how to operate, it will cease to function.

What Causes Furnace Circuit Board Failure?

Transistor failure

Transistors (semiconductor devices used to transport electrical power) can fail over time due to power surges, age, static electricity, or even power surges.

Loose wiring can also cause your furnace’s control board to stop working properly.

Thermal expansion

Over time, the nearby heat of your furnace can cause thermal expansion and cause the solder connections to fracture.

This will cause your control board to stop working.

Failed solder joints

Failed solder joints are another reason why a furnace control board can fail.

If the solders that connect the Molex plug fail, then your circuit board won’t be able to function.

Relay and Switch damage over time

High temperatures can also cause your relays and switches to stop making contact, causing your control board to not function properly.

Stuck switches

Electro mechanical switches can become stuck after periods of downtime like during winter.

You can try to tap on the relay a few times to see if that helps unstick it.

This is a temporary solution as a stuck relay switch will likely lead to control board failure.


Furnaces normally last between 15-20 years.

A gas furnace control board can give out in that period.

Your control board can be susceptible to static electricity and power surges which can lead to the failure of your furnace’s control board.

How Long Should a Control Board Last?

A furnace control board should last 10 to 15 years on average.

This means that during your furnace’s lifetime of 15-20 years, you’ll have to change out the control board at least once.

Where is the Furnace Control Board?

The control board for a furnace is located near the blower motor chamber near the bottom of the furnace.

The control board should be easy to spot as it’s the board with wires attached to it.

Newer furnaces will have an integrated furnace control board, which controls both the ignition and the starting operations of the furnace.

When to Call an HVAC Technician?

If you’ve noticed that your control board indicator light is red or your furnace is acting strangely, then you should call an HVAC technician.

Your control board controls your furnace, and if wired improperly, can damage your furnace or even worse, your home.

A replacement control board costs around $300 to $650 including installation.

If you have a voltage meter, you can try to troubleshoot your control board and see if it’s operating properly.

While it may seem tempting to replace your control board, it can be dangerous if you don’t know what you’re doing.

About Roy Cohen

Roy Cohen has a burning passion for heating, cooling, and ventilation. He aims to help you save money on expensive repairs and bring you the best HVAC products. He has years of experience behind him in HVAC repair and garage maintenance.

You can find Roy at his LinkedIn or his email: [email protected]

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