Why Do Gas Furnace Control Boards Fail?

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A bad control board is not uncommon a couple of times a week during a busy winter of service calls here in the Sacramento Valley. What are the parts on a circuit control board that fail, and why?

The printed circuit board of a furnace is the brains of the whole operation.  It’s the quarterback calling the shots down on the field of high and low voltage circuitry we work on every day.  On this week’s Fox Family Heating and Air Blog, we’ll talk about the parts on the control board that fail and explore some of the most common reasons why. 

What’s Failing?

So, what is failing on these boards?  A slice of silicon 10 years old should be the same composition as a 1-year old board, right?  It seems so.  Regardless, aging systems do begin to give more problems than the newer ones.

Printed circuit boards these days are composed of shrunk down relays and switches mounted on a rigid green board to orchestrate the sequence of operations that start up the furnace and gives us heat.  30 and 40-year-old furnaces we see still out in the field have these relays and switches.  They’re just bigger and sturdier because they’re made from more durable parts.

Last week we discussed how the smaller a control board gets, the weaker the material it’s made of.  The material is thinner, the solder connections are smaller, and the relays are made with tinier pieces of plastic and metal.

Control Board Life Expectancy

Our customers might think a control board should last the lifetime of the furnace.  And I’d say 50% of them do!  But all parts on the furnace control board have a life expectancy, and many things can happen to accelerate the aging process of the parts on that board.

Assuming there’s power to the board, it should function properly. If it’s not, there’s nothing we can do to bring it back to factory specifications.  You can’t make field solder connections right there on the spot that are going to meet any kind of standard the manufacturers set when creating the board.  Different soldering alloys will clash, resulting in a temporary fix at best.

Failed Solder Connections

That brings me to my first common failure on a control board.  I can’t tell you how many times I’ve talked about failed solder connections on the back of a control board.  Molex plugs have stems that are secured to the board and soldered in place to adhere to the metallic circuitry that act as wires do in a house.  If the wire has a beak in it, that circuit isn’t going to work. 

When the backsides of those Molex plugs develop a crack, it makes a gap between the stem that goes through the board to the backside where it meets the circuitry.  Any fractures in that solder connection are going to start creating intermittent abnormalities.  There’s a low voltage Molex plug and a high voltage plug that’s going to be a part of any furnace control we work on.

Thermal Expansion

You might ask, “What makes the solder connections fracture like that?”  Two words, thermal expansion.  Once the solder is applied and forms, it remains a very rigid metal with very little plasticity.  Warmth creates expansion.  And that kind of expansion within the solder joints is going to create a gap between it and the stem it’s supposed to be attached to.  This will cause problems with your boards either now or later. 

When I see a control board that has fractured solder connections on the back of the board, I let the customer know it doesn’t meet factory specs anymore and offer to replace it for them so they don’t have problems in the future.

Relays and Switches can Stick, Burn and Pit

Just like a contactor on the condenser outside, the control board at the furnace has miniature relays which allow certain motors to receive the voltage they need to operate.  And just like the contactor on the condenser, those furnace relays start to pit and burn from arcing that occurs across the contacts as they close.

High temperatures can melt the protective coating on the windings of the coil of a relay, which can prevent the contacts from closing in the first place.  Plastic pieces that the contacts are mounted to can lose stability with ambient heat surrounding the relay, too.  This can warp the contacts of the relay causing them to be misaligned and unable to function properly.

Stuck Switches

When an electro-mechanical switch like the ones on our boards is suddenly being used after a long period of downtime, like the end of one winter to start of the next winter, it can become permanently stuck.  Tapping on the relay can sometimes help, but only delays the inevitable failure of the board.

Over-voltage, like in-rush and other voltage spikes, creates constant overheating.  Under-voltage can prevent the contacts of a relay from staying closed securely.  And it’s not just the voltage that’s damaging these parts.  It’s also the current being carried with that voltage which wears out switches prematurely.


Transistors are typically the first part to fail in a control board.  Once again, the damaging heat and energy that hits those transistors due to voltage spikes, or even a little bit of static electricity, can wear out a board prematurely.  This is going to change the composition of the materials they’re made of.   Over time, they just give out, preventing the control board from working properly.

Power Surges

Asking questions with the homeowner can reveal a lot when diving into an HVAC system that isn’t working properly.  A recent thunderstorm or lightning strikes in the area can send a surge through the house’s electrical system.  That surge might not affect the lights or kitchen appliances in the house.  They may not even trip the breaker if the furnace is on at the main panel.  But it might take out the transformer before the board, sending a jolt to that sensitive control board.

Brownouts from the power company are notorious for damaging HVAC equipment.  A reduction in power that suddenly comes back on with no warning can damage the protective coatings on parts, causing them to fail either now, or even a couple years from now.

Another power surge a house can experience is a car accident in the area that may have taken out a power line. As the connections of those high voltage wires attached to the poles rip apart or get stretched, the influx of energy and the damage it causes happens instantly.  

Many HVAC parts have been taken out by these three situations, causing anywhere from a few thousand dollars’ worth of damage, to simply blowing a little 3-amp fuse on the control board.  No one should ever underestimate the freakish damage that can occur to an HVAC system when power surges happen in or around a house.

Static Electricity  

Careless or unsuspecting technicians who walk across a carpeted floor to get to their furnace can build up more voltage on the body than it can store.  As a result, that voltage will need to be transferred to the next piece of metal it comes in contact with.  You don’t want that to be the metal on a control board.  Electrostatic discharge (ESD) can even develop after you’ve grounded yourself to the furnace the first time.  Standing on a carpet can create that static very easily. 

The damage is done to the control board terminals when ESD hits the board.  There are very thin insulating layers within the control board’s transistors, relays, switches, and solder joints that will break it down.  What’s even worse is that sometimes that discharge won’t cause damage to the board immediately.  It’ll damage the insulation to such a degree that the device fails sometimes hours or even years later.

Control Board Degradation Over Time

A diagnosis of a bad control board is not an uncommon one.  But it makes me wonder what that board has gone through during its life to have gotten to the point where it’s now failed.  The parts themselves have an expected life span.  Everyone agrees with that.  But factors such as thermal expansion, power surges, and static electricity all play a big part in the degradation of a control board over time.

We’ve been getting excellent feedback from our fellow technicians like you who are out in the field working on this stuff everyday.  Please feel free to express your opinions and share your stories about failed control boards in the comments section below.  As HVAC techs, we’re always trying to learn, and there’s no better information than the lessons you’ve learned and can pass on to us.

Thanks so much for stopping. We’ll see you on the next blog post!

Fox Family Heating & Air serves Sacramento, Rancho Cordova, and all of Northern California. If you’re looking to schedule HVAC service in these areas, give us a call!

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