The first cold night of the season is when most people find out their furnace has a problem. In Los Angeles and the foothills, furnaces sit unused for months, and dust, dead batteries and worn parts show up the moment you turn the heat on. Before you call for service, here are the most common reasons a furnace stops working — and what you can safely check yourself.
1. The thermostat is set wrong or has dead batteries
It sounds obvious, but it is the most common “no heat” call. Make sure the thermostat is set to Heat, the set temperature is a few degrees above room temperature, and the fan is on Auto. If the screen is blank or dim, replace the batteries.
2. The furnace switch or breaker is off
Most furnaces have a light-switch style power switch on or near the unit, often in a hallway closet, garage or attic. It is easy to flip it off by accident. Also check the electrical panel for a tripped breaker labeled furnace or FAU.
3. A clogged air filter
A dirty filter chokes airflow. The furnace can overheat and shut itself off as a safety measure, or run but barely heat the house. If your filter is gray and dusty, replace it. After a summer of Santa Ana winds and wildfire smoke, filters in Southern California often clog faster than people expect.
4. The gas supply is off
Check that the gas valve next to the furnace is open (the handle runs parallel to the pipe). If other gas appliances in the home are also not working, contact your gas utility. If you smell gas, leave the home immediately and call the gas company or 911 from outside.
5. A dirty flame sensor
The flame sensor confirms that the burners actually lit. When it is coated with residue, the furnace lights for a few seconds and then shuts off, over and over. Cleaning it is a quick job for a technician and one of the most common fall repairs we do.
6. A failed igniter
Modern furnaces use a hot surface igniter instead of a standing pilot light. Igniters are fragile and wear out over time. If you hear the inducer motor start but the burners never light, a bad igniter is a likely cause.
7. A problem with the blower motor or capacitor
If the burners light but little or no air comes out of the vents, the blower may not be running. A failing capacitor or motor is common in older systems — and the same blower moves air for your AC in summer.
8. Tripped safety limit or pressure switch
Furnaces have several safety switches that shut the system down if something is wrong — overheating, blocked venting or a problem with the draft inducer. These switches are doing their job. The real question is why they tripped, and that needs a proper diagnosis.
9. The furnace is simply worn out
Most gas furnaces last roughly 15–20 years. If yours is in that range, needs frequent repairs, makes new noises or your gas bills keep climbing, it may be time to compare repair against replacement.
What you should not do
- Do not remove panels and work on gas or electrical components yourself.
- Do not keep resetting a furnace that repeatedly shuts off — something is causing it.
- Do not ignore a gas smell, a yellow or flickering flame, or a carbon monoxide alarm.
When to call a professional
If you have checked the thermostat, filter, power and gas and the furnace still does not heat, it is time for a technician. Our heating repair service includes a full diagnosis, a clear explanation of the problem and an upfront price before we start. And to avoid most of these problems next year, schedule a furnace tune-up in the fall.
Need heat tonight?
Orion HVAC serves La Crescenta, Glendale, Burbank, La Cañada Flintridge, Tujunga, Santa Clarita and the San Fernando Valley. Call (323) 736-3737 or book online. You get the price before any work begins, and every repair is backed by our parts and labor guarantee.
