A gas furnace starts in a fixed order, and most no-heat calls in Sunnyvale, Santa Clara and Cupertino are one step of that order failing. The thermostat calls for heat, the inducer fan clears the flue, a pressure switch confirms the draft, the igniter heats up, the gas valve opens, the flame sensor confirms the burners lit, and the blower pushes warm air into the ducts. When a step fails, the control board stops the sequence and usually flashes a code. That code plus a few readings tell a technician where to look. Older furnaces with a standing pilot skip some steps, but the logic is the same.
How we test a furnace that will not heat
- Thermostat demand. Confirm the thermostat is calling for heat and the call actually reaches the furnace.
- Inducer and pressure switch. The inducer must run and the switch must close. A blocked vent, a cracked hose or a plugged condensate trap on a high-efficiency furnace keeps it open.
- Ignition. Watch the hot surface igniter glow or the spark igniter fire, and measure it if it looks weak.
- Flame sensing. Measure the flame signal. A coated sensor is a common reason burners light and then cut out.
- Blower and airflow. Check the blower motor, its capacitor and wheel, the filter, and the temperature rise across the furnace against the range on its rating plate.
- Safety controls and venting. Test the high limit and rollout switches, look for venting problems, and inspect the heat exchanger for cracks, soot or rust.
Symptoms and likely causes
| What you notice | Likely cause | What it usually takes |
|---|---|---|
| Blower runs but the air is cold | Burners not lighting: igniter, gas valve or a tripped limit | Ignition and safety-control testing |
| Burners light, then go out within seconds | Dirty flame sensor or poor ground | Sensor cleaning or replacement |
| Inducer hums but nothing lights | Pressure switch, blocked vent or condensate trap | Vent and switch checks, drain clearing |
| Short cycles on and off | Clogged filter tripping the high limit, closed vents, oversized furnace | Airflow correction before parts |
| Bang or boom at start-up | Delayed ignition from dirty burners or gas pressure | Burner cleaning, gas pressure check |
| Water under the furnace | Condensate leak on a high-efficiency unit | Drain, trap or pump repair |
| Fan never stops | Thermostat fan set to On, a stuck relay or limit | Control testing |
Problems that should not wait
No heat, ignition failure, repeated short cycling, burning smells and safety switch trips all deserve a quick visit and a clear write-up of what was found. Two need extra care. A rollout switch that trips means flame came out of the burner area, often because the heat exchanger or vent is blocked; resetting it again and again is unsafe. And a furnace that keeps tripping its high limit is overheating, usually from poor airflow. If you smell gas or the carbon monoxide alarm sounds, leave the house and call from outside. Phone lines are staffed 24/7, and our emergency service page explains how urgent calls are handled.
Parts matched to your furnace
Before ordering anything, we read the model and serial number from the rating plate. Igniters, flame sensors, pressure switches, inducers and control boards are specific to the furnace, and a part that almost fits can bring the same fault back. Many furnaces also carry a manufacturer parts warranty that depends on age and registration, so if yours may still be covered, that gets checked before you pay for a part. Our repairs are backed by a warranty covering parts and labor. See the brands we service.
Repair or replace?
Age, part availability, the safety condition of the heat exchanger, repair history and warranty status decide it. A failed igniter, sensor, capacitor or switch is a repair on almost any furnace. A cracked heat exchanger is a safety problem, and on an older furnace it usually means replacement. Repeated failures on a unit near the end of its life are the other common reason to look at furnace installation or a heat pump.
Safe checks before you call
- The thermostat is on Heat, set a few degrees above room temperature, with fresh batteries if it uses them.
- The filter is not grey and clogged.
- The furnace door panel is seated. Most furnaces have a door switch that cuts power when the panel is loose.
- The service switch next to the furnace, which looks like a light switch, and the breaker are both on.
- On a high-efficiency furnace, the plastic vent pipes outside are not blocked by leaves or a nest.
If the furnace shuts down on a safety switch, do not keep resetting it. Call instead.
Local notes
In the 1950s–1970s homes common in Sunnyvale, Santa Clara and Cupertino, the furnace sits in a hall closet, the garage or the attic, and attic units are where condensate leaks and tight access show up. Most furnaces here rest from spring to fall, so the first cold night of November is when dusty flame sensors and tired igniters fail. City pages: Sunnyvale, Santa Clara and Cupertino.