The first cold night of the season is when we get the most no-heat calls. The furnace sat all summer, the thermostat gets switched to heat, and nothing happens. Some of those calls need a technician. A lot of them do not.
Two-sentence answer: start with the thermostat, the power, and the filter, because those three cause a large share of no-start calls and you can check them yourself. If the furnace powers up and still will not light, the problem is usually the igniter, the flame sensor, or the gas supply, and that is where you stop and call.
Run the checks below in order. Each one rules out a cause before you move to the next.
1. Thermostat: mode, setpoint, batteries
Check: Set the thermostat to HEAT (not COOL or AUTO) and set it at least 3 degrees above the room temperature. If the display is blank or dim, replace the batteries.
Why it matters: A thermostat that was left in COOL mode all summer, or one with dying batteries, will not send a heat call. Smart thermostats can also lose their schedule after a power outage or a Wi-Fi reset.
Still nothing? Listen at the furnace. If you hear nothing at all after 2-3 minutes, move to power.
2. Power: the furnace switch and the breaker
Check: Most furnaces have a light-switch-style power switch on the wall or ceiling near the unit. It gets flipped off by accident more often than you would think, especially on attic and garage installs. Then check the breaker panel for a tripped HVAC or furnace breaker.
If the breaker trips again right away: leave it off and call. A breaker that will not hold points to an electrical fault in the furnace.
3. The front panel and door safety switch
Check: The furnace blower compartment has a small push-button switch that cuts power when the panel is off. If someone changed the filter and the panel is not fully seated, the furnace will not run. Remove the panel and reseat it firmly.
4. The air filter
Check: Pull the filter. If it is gray and caked, replace it.
Why it matters: A clogged filter chokes airflow, the heat exchanger overheats, and the high-limit switch shuts the burners down. After a few trips, many control boards lock out entirely. High Desert homes load filters faster than most because of Mojave dust, so a filter that was fine in spring can be packed by fall. Our filter change guide covers the right interval for dusty areas.
5. Gas supply (and propane level)
Check: Make sure the gas valve on the line going into the furnace is open (handle parallel to the pipe). If other gas appliances like the stove or water heater are also out, the problem is upstream.
Propane homes: In Phelan, Wrightwood, and parts of Oak Hills and Lucerne Valley, a lot of homes heat with propane. Check the tank gauge. A tank below about 20% can deliver weak pressure in cold weather, and an empty tank means the system has to be leak-checked and relit after refill. Call your propane supplier first, then us if the furnace still will not light.
If you smell gas: Stop here. Leave the house and call your gas utility (Southwest Gas 877-860-6020 in the Victor Valley, SoCalGas 1-800-427-2200 in its areas) or your propane supplier from outside. Our gas leak warning signs post has the full steps.
6. Reset the control board
Check: Turn the furnace power switch off for 30 seconds, then back on. This clears a lockout on most modern control boards. Many boards have a small window with a blinking LED. Count the blinks and match them to the code chart on the inside of the panel door. Take a photo of the code for the technician.
If it starts, runs briefly, and locks out again: the lockout was protecting something. Do not keep resetting it. Move on or call.
7. Igniter (the most common failed part)
Symptom: You hear the inducer fan start, then a click, but no orange glow and no flame. After a few tries the furnace locks out.
What’s happening: Most furnaces built after 2000 use a hot-surface igniter, a ceramic element that glows hot to light the gas. They wear out, and they fail more often in homes that run the heat hard through cold snaps.
Fix: Replace the igniter. $185-$320 installed. This one is a technician job because it is electrical work next to the gas valve.
Older furnaces with a standing pilot have their own failure modes. See why a furnace pilot light keeps going out.
8. Flame sensor
Symptom: The burners light, run for a few seconds, then shut off. The furnace retries two or three times and locks out.
What’s happening: The flame sensor is a thin metal rod that confirms the burners are lit. When it gets coated with oxide, it cannot read the flame and the gas valve shuts off as a safety measure.
Fix: Cleaning or replacement, $185-$285. A dirty sensor is often a sign the furnace skipped its last tune-up.
9. Pressure switch, inducer, and condensate (high-efficiency furnaces)
Symptom: The inducer fan runs but the furnace never lights, or the board shows a pressure switch code.
What’s happening: Before the burners light, the inducer motor pulls air through the furnace, and a pressure switch confirms the venting is clear. On 90%+ furnaces, a clogged condensate drain, a blocked vent pipe, or a failing inducer will keep that switch open. Furnaces in unheated attics and garages can also have condensate lines freeze on the coldest High Desert nights.
Fix: Diagnostic plus repair. Most repairs in this group fall within our typical $150-$650 furnace repair range, and we quote before we start.
Quick reference
| What you notice | Likely cause | DIY? |
|---|---|---|
| Thermostat blank or dim | Batteries | Yes |
| Nothing at all at the furnace | Power switch or breaker | Yes |
| Nothing after a filter change | Door safety switch | Yes |
| Starts, then locks out after overheating | Clogged filter, high-limit | Yes (filter) |
| Click, no glow, no flame | Igniter, $185-$320 | No |
| Lights, then shuts off in seconds | Flame sensor, $185-$285 | No |
| Inducer runs, never lights | Pressure switch, vent, or condensate | No |
| Smell of gas | Gas leak | Leave the house and call |
Why a fall tune-up prevents most of this
Most of the failures on this list give warning signs months before they leave you without heat. On a tune-up we check the igniter, clean the flame sensor, verify gas pressure, test the pressure switch and condensate drain, and run a combustion test. See furnace tune-up pricing. In the High Desert we also confirm the furnace is set up for your elevation, which our altitude guide explains.
When to call
Call 760-983-2326 if you have worked through checks 1 through 6 and the furnace still will not run, or any time you smell gas. See furnace repair for how our service calls work, or furnace repair in Hesperia if you are local to our office.
