What to Do If Your Furnace Won’t Turn On

A furnace that refuses to turn on can be a major frustration – especially when you need warmth on a chilly Southern California evening. While Los Angeles and the surrounding areas may not experience freezing winters, a functional heating system is still essential for comfort. If your furnace won’t start, don’t panic – there are several troubleshooting steps you can take before calling a professional.

At Southwest HVAC, we’ve helped countless homeowners in Downey, Los Angeles, and the rest of the SoCal region diagnose and fix heating issues. Below, we’ll walk you through the most common reasons your furnace may not turn on and what you can do about it.

1. Check the Thermostat

One of the simplest, but most overlooked causes of a furnace malfunction is the thermostat. Before assuming your furnace is broken, take a moment to check the following:

• Is it set to “Heat”? Sometimes, the thermostat may still be set to “Cool” or “Off.” Switch it to “Heat” and raise the temperature a few degrees to see if the furnace kicks on.
• Check the batteries. If your thermostat is battery-powered, weak or dead batteries can prevent it from communicating with your furnace. Replace them and try again.
• Smart thermostat issues. If you have a Wi-Fi-enabled thermostat, make sure it’s connected to your home’s network and hasn’t lost power. A system reboot may be necessary.
• Verify thermostat placement. If the thermostat is located near a heat source, such as a sunny window or an appliance, it may be getting false readings and not activating the furnace when needed.

If your thermostat seems to be functioning correctly, but the furnace still won’t turn on, it’s time to proceed to the next troubleshooting step.

2. Inspect the Power Supply

Even gas furnaces rely on electricity to function. A power issue could be the reason your furnace isn’t turning on. Here’s what to check:

• Make sure the furnace switch is on. Many furnaces have a switch that resembles a light switch, usually located on or near the unit. If you find that it’s been turned off, switch it back to the “on” position.
• Check the circuit breaker. A tripped breaker can cut power to your furnace. Locate your electrical panel, find the furnace circuit, and reset it if needed.
• Inspect the furnace fuse. If your furnace has a blown internal fuse, it may need to be replaced.
• Test the electrical outlet. If your furnace is plugged into an outlet, verify that the outlet itself is working by testing it with another device.

If your furnace still isn’t working, proceed to the next troubleshooting step.

3. Examine the Ignition System or Pilot Light

Older gas furnaces rely on a standing pilot light, while newer models use an electronic ignition system. If there are issues with either, the furnace might not turn on.

For older furnaces: If your pilot light is out, be sure to review the manufacturer’s instructions before relighting it. This is the best way to ensure your own safety.

For modern furnaces: Many newer models have an electronic ignition system that can fail due to a dirty flame sensor or ignition switch. If your furnace has an error code, refer to the manual for troubleshooting steps.

Safety note: If you smell gas, do not attempt to relight the pilot light. Leave your home right away and notify the local gas company.

Check for drafts. If your pilot light frequently goes out, it may be due to drafts in your home, requiring additional insulation or sealing of leaks.

4. Check the Air Filter

A clogged or dirty air filter can block airflow, leading your furnace to overheat and shut down to prevent damage. In SoCal, where air quality can fluctuate, HVAC filters can accumulate dust and debris quickly.

• Inspect your filter. If it’s visibly clogged or dirty, replace it with a clean one.
• Regular maintenance tip: Change your filter every 1 to 3 months to maintain optimal furnace performance.
• Check for airflow blockages. If your vents are covered by furniture or curtains, this could also impact your furnace’s efficiency.

If a clogged filter was the issue, your furnace should turn back on after replacing it.

5. Inspect the Gas Supply

If your furnace isn’t receiving gas, it won’t be able to ignite or produce heat. Here’s what to do:

• Confirm that the gas valve is open. The gas valve should be parallel to the pipe – if it’s turned perpendicular, it’s closed.
• Check other gas appliances. If your stove or water heater is also not working, you may have a gas supply issue. Contact your gas provider for assistance.
• Look for an error code. Some furnaces will display an error message if there’s a gas flow problem.

If the furnace still doesn’t turn on after confirming the gas supply, it may be time to call a professional.

6. Look for Blocked or Dirty Vents

Proper airflow plays a big role in the functionality of your furnace. If vents or registers are blocked, the system may overheat or fail to distribute heat properly.

• Check all supply and return vents. Make sure they’re open and free from obstructions, such as rugs, curtains, or furniture.
• Inspect your ductwork. If your home has excessive dust buildup, duct cleaning may improve airflow and efficiency.
• Check for pests. Small animals or insects can sometimes build nests in ductwork, restricting airflow.

Maintaining clean, unobstructed vents helps your furnace run more efficiently and prevents future issues.

7. Consider the Age of Your Furnace

If your furnace is over 15 years old, it could be nearing the end of its useful life. Older units can experience frequent breakdowns and may not operate efficiently. Signs of an aging furnace include:

• Corrosion or rust on the unit, which can lead to malfunctions
• Yellow pilot light (rather than the usual blue), which could signal a carbon monoxide issue
• Strange noises, such as banging or rattling
• Increased energy bills
• Inconsistent heating or frequent repairs

If your furnace is aging and showing signs of trouble, it might be time to start thinking about replacing it.

8. Know When to Call a Professional HVAC Technician

If you’ve gone through these troubleshooting steps and your furnace still won’t turn on, it’s time to contact a professional. Some issues require specialized tools and expertise, including:

• Strange noises or burning smells coming from the furnace
• Frequent system cycling (turning on and off repeatedly)
• A furnace that turns on, but doesn’t produce heat
• Issues with the furnace’s control board or wiring

The experienced technicians at Southwest HVAC can diagnose and repair any furnace issue quickly and efficiently. Whether you’re in Downey, Los Angeles, or anywhere in Southern California, we’re here to keep your home comfortable year-round.

A non-working furnace doesn’t always mean a major repair is needed. By checking your thermostat, power supply, air filter, and gas connections, you may be able to resolve the issue on your own. However, if the problem persists, it’s best to call in the professionals.

Southwest HVAC provides expert furnace repair and maintenance services throughout SoCal. Our experienced technicians can quickly identify the issue and make your home warm and cozy once again. Don’t wait until you’re left out in the cold – contact us today to schedule an inspection! We’ll get your furnace running smoothly and keep your home warm during the cooler months.