Troubleshooting A Furnace Control Board That Has No Lights

If you’re a staunch DIYer, you’ll know that the control board is the first place to check when your furnace malfunctions. The control board features several LEDs designed to light up in pre-defined patterns, depending on the malfunction. 

For instance, some control boards will blink twice to indicate a blocked filter and perhaps four times to signal a missing air filter. The lights blink for nearly all other furnace issues, from low fuel pressure (maybe due to an empty propane tank) to overheating, perhaps due to flame rollout in the gas chamber. 

Unfortunately, the control board itself can malfunction, potentially affecting the onboard LED lights. This can create a massive problem because the lack of LED lighting makes furnace troubleshooting and diagnosis extremely challenging. 

So, what can cause the LED lights to go out, and what can you do about it? Read on to find out. 

What is a Furnace Control Board?

The furnace control board is essentially the brain of the furnace. It coordinates critical functions of the furnace, controlling everything, from fuel pressure to the level of heating. 

There are two types of furnace control boards;

Simple ignition control board module

Simple ignition furnace control boards are classical control board modules found in older heating systems. Typically, they operate the gas valve, ignition system, and flame sensing circuitry. 

Although they work just fine, they tend to be less efficient. They are also a little difficult to diagnose. 

Integrated furnace control board module

Integrated furnace control board modules are modern control board modules found in newer furnaces. 

In addition to the gas valve, ignition system, and flame sensing circuitry, the integrated furnace control board controls furnace blowers and digital circuitry, including remote control and Wi-Fi connectivity. 

What Does the Control Board Do?

The furnace control board operates almost everything that goes on within the furnace. More specifically, it handles the following; 

  • Electronic ignition
  • Opening the gas valve
  • Checking the burner unit
  • Activating the blower
  • Shutting off the gas
  • Venting the heat exchanger 

Modern furnace control boards handle other integrated HVAC capabilities, such as air conditioning in mini-split systems, dehumidification, and fan control. 

It’s easier to understand the control board’s range of functions when you understand how it works. The furnace control board receives input from your thermostat and furnace sensors, analyses the data, and makes critical decisions directly impacting your heating (and air conditioning). Then it distributes the final decisions to various components of the furnace to implement the instructions. 

However, it’s more than a relay system. Modern control boards are designed to monitor sensor input for consistency to flag signs of trouble. This enhances appliance efficiency. For instance, the control board can use data from the sensors to pre-empt a gas valve leak, thus prevent gas poisoning and perhaps a fire. 

Additionally, heating system control boards store error codes whenever a sensor reports alarming incidents, such as a part failure. You (or your technician) can later retrieve this information to assist with troubleshooting and repair. 

Surprisingly, the control board even powers some components of the furnace, notably the in-built thermostat. Built-in thermostats don’t draw power directly from the home’s supply. Instead, a transformer on the furnace converts 120V power from the home’s supply to 40V, which flows via the control board to the thermostat. 

Reasons Why Your Furnace Control Board Has No Lights

If you find that your furnace control board has no lights, there’s likely a problem. Check again to make sure the unit is plugged ON. If so, you may have one of the following circuit board situations;

1. The board is too old

Many customers think that the furnace circuit board should last a lifetime. Well, some do. However, more than half don’t. So, before you embark on expensive repairs, consider the age of your furnace. Could the control board have reached its end of life? Is it time to replace it? Sometimes it’s the most sensible solution – financially. 

2. Failed solder connections

The furnace control board might also fail to light up if the soldier connections are loose or failed. Molex plugs have stems secured to the board and soldered in place to adhere to the metallic circuitry. If the wire breaks, the circuit will fail, and the LED lights will remain off too. If you’re wondering, soldier cracks are quite common. 

3. Thermal expansion 

The most common reason for soldier cracks is thermal expansion. When solder is applied and secures, it forms a rigid metal with limited elasticity. Unfortunately, circuit boards become warm when you’re running the furnace. This warmth can cause expansion within the solder joints, creating gaps between it and the attached stems. This can cause problems to the board immediately or in due time. 

4. Burnt relays and switches 

High temperatures on the circuit board can also burn the miniature relays that transfer information and power to the different components, such as motors, on the board. Moreover, the heat can melt the protective coatings shielding the relay coils, preventing contacts on the board from closing. Or the contacts may become misaligned. All these can interfere with board function. 

5. Failed transistors 

Transistors are often the first part to fail on the control board. Transistor failure can be due to many things. For instance, voltage spikes can completely damage a transistor. Static electricity is another common culprit. Studies show that static electric discharge can damage insulation to the control board’s transistors, relays, and solder joints immediately, within days, or over many years. 

How to Test the Furnace Control Board

Fortunately, testing the furnace control board. Ideally, we’d recommend that you hire an HVAC pro for greater accuracy. However, there’s nothing wrong with preliminary testing to confirm that the board isn’t getting or transmitting power.  

The purpose of this test is to verify that power is traveling from the home’s supply to the control board and from the board to the other furnace components, then back. 

Safety Disclaimer: The furnace operates on natural gas, propane, or oil. It also uses electricity to power various components. Therefore, utmost caution is a must. 

What You Need

  • A multimeter 
  • Screwdriver 
  • Wrenches 
  • Test leads 
  • 12-volt power source
  • Your furnace manual

Step-by-Step Guide 

To check if the circuit board is getting power

  1. Get your multimeter and set it to DC
  2. Attach one lead of the meter to the ground
  3. Connect the other lead to the connection where the power connects to the circuit board
  4. The reading should be 120V

To check the relay on the circuit board

  1. Attach the first multimeter lead to the ground
  2. Connect the other lead to the connection after the relay that leads to the blower motor
  3. With the thermostat ON, you should get the same reading as above. It shows that power is traveling past the relay to the blower motor. 

Check that power is flowing back to the circuit board

  1. Locate the connections on the circuit board where power returns from the sail and high-level switches. 
  2. After verifying that the thermostat is ON, check the reading on the multimeter
  3. It should be 120V

Besides voltage tests, use your eyes to check for signs of trouble. For instance, you should be able to tell if the printed relays are scratched or melted. If so, then the circuit board may malfunction.

How to Reset a Furnace Control Board

Sometimes resetting the furnace control board can solve the lighting problem and return your furnace to normal function. This is especially true if the issue was caused by high voltage and the control board shut down to protect the internal components. Resetting the board can fix the issue.

 You don’t need any special tools to reset the furnace control board. But you need to be highly cautious, keeping in mind the dangers of gas and electricity. Proceed as follows;

  1. Flip the circuit breaker switch into the OFF position to shut off power
  2. Allow the gases in the home to dissipate
  3. Wait at least a full minute
  4. Turn the heater back ON
  5. Set the thermostat to the desired temperature 
  6. Watch to see if it works 

Replacing the Furnace Control Board

If neither resetting nor repairs work, it might be time to replace your furnace circuit board. This is a strictly expert-only job. However, you need to know a few things. 

For one, selecting a replacement furnace control board is a critical process. You want the exact match. Don’t accept a look-a-like or something that seems close to the original.

It creates a massive risk of fire. But even before that, many furnaces will reject the wrong circuit board, potentially costing you hundreds of dollars in losses. 

Secondly, always insist on original replacement parts from the manufacturer. Third-party replacement parts might be cheaper. But they come with a massive safety risk and rarely last a few years. 

Summary

Furnace board control board malfunctions don’t happen often. You’ll likely experience one after every few years. However, they can be extremely frustrating when they happen and often necessitate expensive repairs or a complete replacement. 

Be sure to test your board before you call the pros thoroughly and, if you need to replace or repair it, insist on genuine parts and professional workmanship.