Skip to Content

Furnace Not Starting? 4 Common Causes (and What To Do)

A furnace relies on different parts and a power source, to function. Once turned on, ignition occurs, followed by the blower distributing air through the ducts that run through the building. When the furnace isn’t starting, it means there’s a problem with your furnace or power supply. 

A furnace not starting is caused by a faulty power supply or control board, problems with the ignitor, or a triggered safety switch. Fixes include investigating the power supply, checking the control board, fixing or replacing the ignitor, and troubleshooting the safety switch.

This article will teach you how to diagnose a problem with the furnace not starting. I’ll also tell you how to fix the most common causes. 

The Power Supply Is Faulty

Your furnace and the connected hardware all depend on efficient power to operate. This is why the diagnosis process should always begin with the power supply attached to your HVAC system. 

A faulty power supply can cause issues with the furnace, the thermostat, and other hardware that form your HVAC.

The power supply of the furnace can run into a few problems. For instance, the cables that help distribute power to the different parts of the system may be to blame. In other cases, components of the furnace itself fail to distribute the power. 

If the furnace isn’t starting, there is a good chance that the cause may lie with the power supply. 

How To Fix

You should inspect the different parts of the power supply and its related cabling. Having a voltage meter is very useful when testing the power supply. 

Begin by looking at the cables used in the system. Start at the primary power supply and inspect the cable as it runs toward the furnace. Check for signs of physical damage, burnt areas, or a piece of the cover that has started to come off. 

Test multiple points of the power supply to see if power is lost somewhere specific. 

To fix a problem with the power supply, consider getting the cables replaced. This can help if the issue lies with damaged cables. 

Here’s a video to help you diagnose a power problem with your furnace: 

The Control Board Is Faulty

The control board plays an important role in the furnace. It sends signals and power to the appropriate channels. When you switch the furnace settings, the control board ensures your new settings are implemented. 

Problems with the control board can lead to several issues. This may include problems with heat adjustments, the blower not starting, or even the ignitor not functioning. So, a faulty control board is often the reason behind a furnace not starting. 

How To Fix

The control board is complex, but it should be relatively simple to troubleshoot. You’ll need to test specific points of the board and look for damage. 

Follow these steps before deciding whether the control board is to blame:

  1. Open up the main door on the furnace. This should give you access to the control board. If you’re not sure where the control board is or how it looks, refer to the owner’s manual. 
  2. Take a look at the control board. If lighting is bad, use a flashlight. You want to see if there are any signs of physical damage to the board. This can happen during a power surge, for example. There will be a dark mark on the board, which is a sign that the board is burnt. 
  3. Test the power of the control board. Get a voltage meter for this. The voltage meter should be placed at different connectors on the board. Make sure the meter shows a stable reading at all of these connectors. 

If the control board is faulty, you need to get it replaced. The process of replacing a control is tricky as it requires skill and expertise. Ensure the new control board you buy is compatible with the furnace model in the building. 

Problems With the Ignitor

The ignitor on a furnace creates a spark. After the spark, the burner produces and maintains the flames. These flames help heat the air in the furnace, which is then distributed through your ductwork. 

There’s a chance that the furnace not starting is related to issues with the ignitor. The ignitor may be faulty, which means it won’t create a spark. When there is no spark, flames cannot be produced. This means the burner cannot heat the air. 

The power supply to the ignitor could also be the problem in this situation. 

How To Fix

Start by looking at the ignitor — make sure it glows when the furnace is turned on. If it’s not glowing, that’s a sign that the ignitor is faulty. Note that this could also be a sign that there is no power going to the ignitor, however. 

In that case, you need to open the furnace. Take safety precautions to ensure you don’t get shocked. Turn the furnace off before you work on it. 

Remove the ignitor to inspect it more closely. You can put the ignitor in a relative’s or neighbor’s furnace to see if it works. 

Alternatively, borrow an ignitor from a different furnace and test it. If it works, then you need to replace your ignitor. 

The Safety Switch Was Triggered

Depending on your furnace model, you may have a safety switch. This switch is generally located at the door. When the door is shut, the safety switch is turned off. This means the furnace can operate normally.

But if the door isn’t shut properly, the safety switch is triggered when you turn the furnace on. It won’t let the furnace start to prevent a fire hazard. 

How To Fix

You first need to know if the problem is caused by the safety switch triggered for a reason or if it’s just faulty.

Check the furnace door to ensure it’s properly closed. If the door isn’t clipped into place, then this might be why the safety switch is triggered. 

When the door is properly closed, but you still see a red light at the safety switch notification LED, then it may signal problems with the switch itself. Thankfully, a replacement switch won’t be expensive.

Here’s how to do it:

  1. Turn off the furnace.
  2. Open the furnace.
  3. Remove the existing safety switch by referring to the owner’s manual.
  4. Get an identical replacement.
  5. Install it.
  6. Check to see if it works.

This video walks you through the steps to replace a door switch:


When there’s a problem with the control board or power supply, the furnace won’t start correctly. I hope you now understand how to diagnose this issue and fix the problem. 

To ensure you don’t cause further damage, consider getting an HVAC expert to look at the furnace.


  • Steve Rajeckas

    Steve Rajeckas is an HVAC hobbyist with an avid interest in learning innovative ways to keep rooms, buildings, and everything else at the optimal temperature. When he's not working on new posts for Temperature Master, he can be found reading books or exploring the outdoors.

As an Amazon Associate, we earn from qualifying purchases. We may also earn commissions if you purchase products from other retailers after clicking on a link from our site.