When a furnace in Fremont stops heating, runs in short bursts or blows cool air, the cause is usually one weak link in its start-up sequence (an igniter, a flame sensor, a pressure switch) or an airflow problem, such as a clogged filter that makes the furnace overheat and shut itself off. A proper diagnosis finds which one before any part is replaced. We repair gas and electric furnaces, and the occasional oil-fired unit, in homes across Fremont, Newark and Milpitas.
How a gas furnace starts, and where it fails
Knowing the sequence makes the symptoms easier to read. When the thermostat calls for heat:
- The inducer fan starts and pulls air through the vent.
- A pressure switch confirms the vent is drawing properly.
- The igniter heats up (or sparks) and the gas valve opens.
- The flame sensor proves there is a flame. If it cannot, the gas valve closes within seconds.
- After a short delay the blower starts and pushes warm air through the ducts.
- Limit switches shut the burners off if the furnace overheats.
A failure at any step stops the cycle, and most modern furnaces report it as a blinking light code on the control board. If you can see the light through the small window in the furnace door, write down the pattern before you call. It tells the technician where to start.
Common symptoms and what they usually mean
| What you notice | Likely cause | What it usually takes |
|---|---|---|
| Burners light, then go out after a few seconds | Dirty flame sensor or a weak flame | Sensor cleaning or replacement, burner check |
| Inducer runs but nothing lights | Cracked igniter, pressure switch, blocked vent or condensate trap | Testing each step of the sequence |
| Runs in short bursts, house never warms up | Clogged filter or blocked returns tripping the high limit | Airflow correction, limit switch test |
| Blower runs but the air is cold | Burners not lighting, or the thermostat fan set to On | Ignition diagnosis or a thermostat setting |
| Nothing happens at all | Power switch or breaker off, door switch, blown fuse, transformer or thermostat | Electrical checks |
| Loud bang when the burners light | Delayed ignition from dirty burners or a weak igniter | Burner cleaning and gas check; keep it off until then |
| Squeal or grinding from the blower | Worn motor bearings or a failing blower motor | Motor service or replacement |
| Some rooms warm, others cold | Crushed or disconnected ducts in the attic or crawlspace | Duct inspection |
Every repair ends with a safety check
A furnace that lights is not automatically a furnace that is safe. After the repair we run it through full cycles and check the flame, the venting and draft, the safety switches, the blower and the temperature rise across the furnace against the range on its label. If we find a cracked heat exchanger, we show it to you and recommend the furnace stay off: a crack can let combustion gases into the air you breathe. That is the one case where “repair first” does not apply.
Repair or replace?
Repair makes sense when one part has failed and the rest of the furnace is sound, which is true of most no-heat calls. Replacement is worth pricing when the heat exchanger is cracked, when an older furnace needs a major part such as the inducer or the control board after several earlier repairs, or when you were already thinking about adding cooling. You see the repair price before any work starts. If a new system is the better choice, it comes with a free in-home estimate: see furnace installation, or consider a heat pump, which heats and cools with one system.
Before you call: safe things to check
- The thermostat is set to Heat, a few degrees above room temperature, with fresh batteries if it uses them.
- A blank smart thermostat often means the furnace lost its low-voltage power (a tripped breaker, an open service switch or a blown fuse on the control board), not a broken thermostat. Its app may also show when heating last ran, which is useful to mention when you call.
- The filter is not grey and packed with dust.
- The furnace’s service switch (it looks like a light switch on or near the unit) and its breaker are on, and the front panel is fully closed. Most furnaces will not run with the panel off.
If the burners light and go out repeatedly, do not keep restarting the furnace. Let us find the cause.
Fremont notes
Furnaces here sit idle most of the year and then run from November to March, so the first cold night is when weak igniters and dusty flame sensors show up. Many 1950s–1980s homes in Fremont and Newark have the furnace in a hall closet, the garage or the attic, with ducts running through the attic or crawlspace. Townhomes in Milpitas often fit the furnace into a compact closet, where a blocked return or a clogged filter trips the high limit quickly. A fall maintenance visit catches most of these problems before the heating season starts.
For city-specific details, see furnace repair in Fremont, Newark and Milpitas. If you smell gas or your CO alarm sounds, read our emergency HVAC page first.