If your furnace is not turning on, the cause is usually something small: a thermostat set to the wrong mode, a tripped breaker, a clogged filter, or a door panel that is not seated. Work through the seven checks below in order. Most take under five minutes, and none requires opening anything that holds gas or high voltage.
Los Angeles furnaces sit idle for eight or nine months, then get asked to start on the first cold night in November or December. Dust settles on the burners, the filter has been loaded since the last Santa Ana, and a thermostat battery may have died in July. That is why a heater that is not working here often turns out to need one simple fix.
Before you start: what a furnace not turning on looks like
People describe the same problem a few ways, and the symptom helps narrow the cause. Use this to orient yourself.
Nothing happens at all, no click, no fan, no light on the thermostat. That points to power: the thermostat, a breaker, or the furnace switch.
The fan runs but the air is cold. The furnace is not lighting, which points to the ignitor, the gas supply, a safety lockout or a condensate switch. A furnace blowing cold air is also normal for the first 30 to 90 seconds of a cycle, so give it a minute before deciding.
It clicks, tries to start, then shuts down, or runs a few minutes and stops. That is a safety system doing its job, usually over airflow, a dirty flame sensor or a failing ignitor.
If you have a heat pump instead of a gas furnace, checks one, two and three still apply, and the gas and ignition checks do not. A heat pump not heating has its own quirks, covered in the questions at the end.
Seven checks for a furnace not turning on
1. Thermostat mode, setpoint and batteries
Start at the wall. Confirm the thermostat is set to Heat (not Cool, Off or Auto with a cooling setpoint), and set the target at least 5 degrees above the room temperature so it has a reason to call for heat.
If the screen is blank or dim, replace the batteries. Many thermostats run on two AA or AAA cells, and a weak pair can show a display while being too weak to close the relay that tells the furnace to start. Smart thermostats can also lose their schedule or Wi-Fi after a power blip, so look for a "hold" or "away" setting that is overriding your setpoint.
Give it a couple of minutes after any change, because some systems delay before firing.
2. Breaker and furnace power switch
Look at your electrical panel for a tripped breaker, which sits in the middle position between on and off. Flip it fully off, then on once. If it trips again right away, stop there and do not keep resetting it. A breaker that will not hold is telling you there is a short or a failing motor, and repeated resets can damage wiring.
Then find the furnace switch. It looks like an ordinary light switch and is mounted on or near the furnace, often on a wall or the side of the unit in a garage, closet or attic. They get switched off by accident more often than you would expect, usually by someone who thought it controlled a light. Make sure it is on.
3. The air filter
A clogged filter is the most common cause of a furnace that starts and then quits. When airflow drops, the heat exchanger overheats and a limit switch shuts the burners off to protect the unit. After a long idle stretch the filter may also hold a summer of Valley dust.
Slide the filter out and hold it up to a light. If you cannot see light through it, replace it. The arrow on the frame points toward the furnace, in the direction air flows. Our seasonal maintenance checklist covers how often that should happen here.
4. The furnace door panel and its safety switch
Furnaces have a small switch that cuts power whenever the front panel is off or not fully seated. If you changed the filter and put the panel back, or if the unit was recently serviced, the panel may be sitting a quarter inch off. Remove it, set it back square, and make sure it clicks in or the latches or screws are fully engaged.
The switch is a small plunger near the door opening that the panel presses in. Never tape it down or bypass it, because it keeps a spinning blower and live wiring away from hands.
5. Gas valve, pilot light or ignition
Check that the gas valve near the furnace is open. The handle should run parallel with the pipe. If it is crosswise, the gas is off.
Furnaces made in the last couple of decades almost never have a standing pilot flame. They use a hot surface ignitor or a spark ignitor. If you can watch through the sight glass, you will hear the inducer fan start, then a glowing element, then the burners light. If the ignitor glows and no flame follows, the gas valve may not be opening. If it never glows, the ignitor has probably failed. A furnace ignitor not working is a common repair on units that have been through many winters. Replacing one is routine for a technician and not something to attempt blind.
An older furnace with a standing pilot can sometimes be relit by following the instructions printed on the unit. If the pilot will not stay lit after one careful attempt, stop. A thermocouple may be worn or the gas supply may be restricted. Do not try repeatedly, and do not light anything if you smell gas (more on that below).
6. Condensate drain and float switch on high efficiency furnaces
A furnace rated 90 percent efficient or higher cools its exhaust until it condenses, which produces water. That water drains out through a small line, and a safety float switch shuts the furnace down if the line backs up. If your furnace has white PVC pipes going out through a wall or the roof, it is this type.
Look for a trap, a clear tube or a small collection pump near the furnace. If water is standing in it, or you see a puddle underneath, the drain is likely clogged with sludge or a kinked hose. After months of disuse that is common. You can clear a clear tube and trap gently and reseat it, but if the pump is not running or water keeps coming back, call someone. Ceilings are expensive.
7. The blinking status light
Most modern furnaces have a small LED visible through a window in the door, or on the control board. It blinks a pattern that corresponds to a code printed on a label on the inside of the panel. One flash may mean normal operation, while a fixed number of flashes might indicate a pressure switch fault, a flame sense failure or a lockout from overheating.
Write the code down before you call, since it saves a technician a lot of guessing. Do not clear the fault repeatedly by cycling the power. A lockout is the furnace protecting itself.
When to stop and call, no matter what
Some situations are not troubleshooting problems. Treat them as a stop sign.
You smell gas
Leave the home immediately. Do not flip switches, use a phone inside, light anything, or try to find the source. Once you are outside and away from the building, call SoCalGas at 1-800-427-2200, or 911 if you cannot reach them. This is SoCalGas's own guidance on their natural gas leak page, which says to evacuate and call from a safe location.
The breaker keeps tripping
One trip after a power flicker is normal. A breaker that trips again as soon as the furnace tries to start points to a failed blower motor, a shorted component or damaged wiring. Leave it off and call.
You smell something burning
A short dusty smell on the first run of the season is normal. A sharp electrical smell, a chemical odor or smoke is not. Our guide to a heater burning smell explains which smells can wait and which mean shutting the unit off now.
Carbon monoxide risk and a cracked heat exchanger
The heat exchanger is the metal chamber that keeps combustion gases separate from the air that goes into your rooms. When it cracks, those gases, including carbon monoxide, can enter the air. You cannot see a crack without taking the furnace apart, so the safeguards are working CO alarms and a regular inspection. The Consumer Product Safety Commission recommends a yearly professional inspection of fuel-burning heating systems, including furnaces, and says heating equipment is among the top contributors to CO deaths. The CPSC also advises battery-operated or battery backup CO alarms on every level of the home and outside sleeping areas.
If an alarm sounds, or anyone has headaches, nausea or dizziness that improves when they leave the house, go outside and call 911. Do not wait for a repair appointment.
What a repair call looks like
If the checks above did not solve it, the usual suspects are a bad ignitor, a dirty flame sensor, a failed pressure switch, an inducer motor, a control board or a gas valve. A technician with a meter narrows it down quickly.
Our technicians are EPA certified W-2 employees, and you get a written estimate before any work begins. To see what the first visit covers, read how a diagnostic fee works. We offer same day service in most of Los Angeles, with no overtime fees, so the diagnostic fee is the same at 2 AM as at 2 PM. We are open 24 hours Sunday to Thursday, Friday 9 AM to 3 PM, and closed Saturday. For a furnace out on a cold night, that matters, and so does knowing about our emergency HVAC service before you need it.
We also will not push a replacement when a repair will do. An ignitor or a sensor on a furnace that is otherwise sound is a repair, and we will say so. You can see what we handle on our heating repair page.
Keeping it from happening next November
Run the heat for 15 minutes on a mild day in October, while every contractor in the county still has open appointments. Change the filter, replace the thermostat batteries, and check your CO alarms (the manufacture date is printed on the back, and they expire). A yearly HVAC maintenance visit catches a weak ignitor or a dirty flame sensor before it strands you on the one cold weekend of the year.
Common questions
Why is my furnace blowing cold air?
Some cold air at the start of a cycle is normal while the burners light and the heat exchanger warms. If it continues, the furnace may be failing to ignite, locking out on overheating from a dirty filter, or your thermostat fan setting may be on "On" instead of "Auto," which blows unheated air between cycles. Check the filter and the fan setting first, then call if cold air persists.
Why is my heat pump not heating?
First confirm the thermostat is on Heat and not Emergency Heat or Cool, and check the filter. In cold weather a heat pump outdoors may run a defrost cycle and blow cooler air for a few minutes, which is normal. If the outdoor unit is iced over for a long time, it is not running, or the air stays cool, a technician needs to check it.
Why won't my furnace turn on even though the thermostat is on?
When a furnace won't turn on with the thermostat calling for heat, the most common reasons are a tripped breaker, the furnace switch turned off, a loose door panel, a clogged filter or a failed ignitor. A condensate switch on a high efficiency furnace can also stop it. Work through the seven checks above, and call if the furnace still will not start.
How do I know if my furnace ignitor is not working?
On a hot surface ignitor, you should see it glow orange before the burners light. If the inducer fan runs but you never see a glow, or it glows and the burners never ignite, the ignitor or the gas valve is the likely fault. The furnace may also click several times and shut down with a blinking error code. Ignitors wear out, and replacing one is a quick, inexpensive repair for a technician.
Furnace still dead? Call us
If you have run these checks and the heat is still off, we offer same day service in most of Los Angeles.
Cool Breeze LA serves homeowners across Los Angeles, Calabasas, Sherman Oaks, Glendale, Pasadena and Burbank. Call (310) 469-1979 or request service online.