Furnace Not Turning On? 7 Things to Check Before Calling for Repairs
There is nothing worse than waking up on the first cold morning of the season, adjusting your thermostat, and realizing your furnace is not turning on. In the San Fernando Valley the temperature might not drop to dangerous levels very often, but a house without heat in December or January is still deeply uncomfortable, and the problem rarely fixes itself.
At E & A Mechanical, our technicians respond to dozens of "no heat" calls every fall and winter across Tujunga, Glendale, Burbank, and the greater Los Angeles area. Over our 25+ years of service we have noticed that many of those calls trace back to a handful of common issues, some of which homeowners can resolve on their own in a matter of minutes. Below we walk through seven things to check before you pick up the phone for an emergency furnace repair call.
1. Thermostat Settings and Power
It sounds too simple, but the thermostat is the single most common reason a furnace won't ignite. Before you do anything else, verify three things:
- Mode. Make sure the thermostat is set to "Heat," not "Cool" or "Off." This is especially easy to overlook in Southern California, where many homeowners switch between heating and cooling within the same week.
- Temperature. Set the target temperature at least three to five degrees above the current room temperature. Some thermostats have a built-in delay and will not send a call for heat unless the gap is large enough.
- Power. If you have a battery-operated thermostat and the screen is blank or dim, replace the batteries. For hardwired models, check that the thermostat is seated firmly against the wall plate.
If the thermostat display is completely dead and batteries are not the issue, the problem may be with the low-voltage wiring between the thermostat and the furnace. That is a job for a licensed technician.
2. Air Filter Clogs
A severely clogged air filter is one of the leading causes of furnace shutdowns. Modern furnaces are equipped with a high-limit safety switch that shuts the system down when the heat exchanger overheats. A dirty filter restricts airflow across the heat exchanger, causes the temperature inside the cabinet to spike, and triggers that safety switch.
Pull the filter out and hold it up to a light source. If you cannot see light through the media, the filter needs to be replaced. In our experience, homeowners in areas with older homes and mature landscaping, such as La Crescenta and Montrose, tend to see filters clog faster because of higher ambient dust levels. We recommend checking the filter monthly during the heating season and replacing it at minimum every 90 days.
If you replace the filter and the furnace still will not start, the high-limit switch may need to be manually reset. Check your owner's manual or call a professional.
3. Circuit Breaker and Power Switch
Even gas furnaces rely on electricity to power the blower motor, control board, and ignition system. Two electrical components can silently cut power to the unit:
- The circuit breaker. Find your electrical panel and look for the breaker labeled "Furnace" or "HVAC." If it has tripped to the middle position, flip it fully to "Off" and then back to "On." If it trips again immediately, do not keep resetting it. Repeated tripping indicates a short circuit or an overloaded motor, and continuing to reset the breaker is a fire hazard.
- The furnace power switch. Most furnaces have a dedicated on/off switch mounted on or near the unit. It often looks like a regular light switch and can easily be bumped to the off position during storage or cleaning in the utility closet. Make sure it is in the "On" position.
Safety note: Never remove the furnace access panels while the power is on. Always shut off the dedicated furnace switch and the breaker before opening the cabinet.
4. Pilot Light or Ignitor Problems
How your furnace lights its burners depends on its age and design:
- Standing pilot light (older systems). If your furnace has a small, continuously burning flame, look through the access window to see if it is lit. If it has gone out, you can usually relight it by following the instructions printed on the furnace label. Turn the gas valve to "Pilot," hold down the reset button, and use a long match or a barbecue lighter. If the pilot will not stay lit after you release the button, the thermocouple is likely worn out and needs to be replaced by a technician.
- Hot surface ignitor (modern systems). Most furnaces built in the last 20 years use a hot surface ignitor, a small silicon carbide or silicon nitride element that glows red-hot to light the gas. These ignitors are fragile and have a typical lifespan of three to five years. If you hear the furnace click on and the inducer motor runs but no flame appears, a failed ignitor is the most likely cause. Replacing an ignitor is a straightforward repair for a licensed HVAC professional.
If you smell gas but the burners never light, shut the furnace off immediately, leave the house, and call your gas utility and then a qualified HVAC technician.
5. Gas Supply
No gas means no heat. Two things can interrupt the gas supply to your furnace:
- The gas valve on the furnace. There is a manual shutoff valve on the gas line feeding the furnace. The handle should be parallel to the pipe, meaning it is open. If it is perpendicular, the valve is closed.
- The main gas meter. If none of your gas appliances are working (stove, water heater, dryer), the issue may be at the meter or with your utility service. Contact SoCalGas to confirm there is no outage or service interruption in your area.
Never attempt to work on gas piping yourself. If you suspect a gas leak, indicated by a rotten-egg smell, hissing near a pipe, or dead vegetation around an outdoor line, evacuate and call 911 or SoCalGas at 1-800-427-2200.
6. Condensate Drain Line Blockage
High-efficiency condensing furnaces (90% AFUE and above) produce water as a byproduct of combustion. That water exits through a condensate drain line, usually a small PVC pipe that runs to a floor drain or condensate pump. If the line becomes clogged with algae, sediment, or debris, a pressure switch detects the backup and shuts the furnace down to prevent water damage.
Check the drain line for visible blockages. You can often clear a minor clog by pouring a cup of distilled white vinegar into the drain opening. If the line is severely blocked or the condensate pump has failed, a technician can flush and restore the system quickly.
7. Safety Switches and Lockout Codes
Modern furnaces have multiple safety switches beyond the high-limit switch discussed above. These include flame sensors, pressure switches, and rollout switches. When any one of them detects an abnormal condition, the control board locks the furnace out and typically flashes a diagnostic LED code through the small window on the lower access panel.
Count the number of flashes and note the pattern (for example, three short flashes followed by a pause). Then consult your owner's manual or search the manufacturer's website for the meaning of that code. Common lockout causes include:
- A dirty flame sensor that cannot confirm the burner lit
- A blocked flue or exhaust vent
- A failed pressure switch due to a cracked inducer hose
Some lockout conditions clear themselves after a set waiting period. Others require a manual reset (turning the furnace power off for 30 seconds and then back on). If the unit keeps locking out after a reset, the underlying issue needs professional diagnosis.
When to Call a Professional for Furnace Troubleshooting
The steps above cover the safe, homeowner-friendly checks. You should call a licensed HVAC technician if:
- You smell gas at any point during your inspection.
- The breaker trips repeatedly.
- The furnace locks out more than once after resetting.
- You are unsure about relighting a pilot light or handling any gas component.
- The ignitor, flame sensor, or pressure switch needs replacement.
At E & A Mechanical, we carry common replacement parts on our service trucks so that most furnace repairs can be completed in a single visit. We serve Tujunga, Glendale, Burbank, Pasadena, Sunland, and the entire San Fernando Valley, and we offer emergency furnace repair for situations that cannot wait.
If you would rather prevent these problems entirely, consider scheduling an annual heating tune-up before the cold season begins. Our technicians inspect ignitors, clean flame sensors, test safety switches, and verify gas pressure so that your furnace is ready when you need it.
Frequently Asked Questions
Why does my furnace turn on but blow cold air?
If the burners never ignite but the blower runs, the ignitor or flame sensor is likely at fault. If the burners do ignite but the air still feels lukewarm, a dirty filter restricting airflow or a failing blower motor may be the cause. Either way, a technician should inspect the system.
How often should I replace my furnace filter?
We recommend checking the filter every 30 days during the heating season and replacing it at least every 90 days. Homes with pets, construction dust, or older ductwork may need more frequent changes.
Is it safe to relight a pilot light myself?
In most cases, yes, as long as you follow the manufacturer's instructions and do not smell a strong gas odor before you begin. If the pilot will not stay lit, do not continue trying. Turn off the gas valve and call a professional.
What does a flashing red light on my furnace mean?
The flashing LED is a diagnostic code generated by the furnace's control board. The number and pattern of flashes correspond to specific faults. Check your owner's manual for the code chart, or contact us and describe the pattern so we can advise you.
Should I repair or replace an old furnace that keeps breaking down?
As a general rule, if the repair cost exceeds 50% of the price of a new furnace, or if the unit is over 15 years old and requiring frequent service, replacement is usually the smarter investment. We offer free estimates to help you compare your options.
This blog is for informational purposes only. HVAC work involving electrical, gas, or refrigerant systems should always be performed by a licensed professional. Attempting repairs without proper training can void warranties and create safety hazards.
Need HVAC service? Schedule service today or call 818-988-9060 for a free estimate.