A gas furnace that blows cold air, clicks without lighting or shuts off after a minute usually has one failed part in its start-up chain: the igniter, the flame sensor, the inducer motor or its pressure switch, a limit switch or the control board. Electric furnaces fail elsewhere: heating elements, sequencers and breakers. In San Jose many of these faults surface on the first cold evening of fall, after the furnace has sat unused since spring.
Gas smell or CO alarm: do this first
- Gas smell (rotten eggs): do not touch light switches, the thermostat or the furnace. Get everyone out of the house, then call the gas utility or 911 from outside.
- Carbon monoxide alarm: get everyone out into fresh air and call 911 from outside. Do not run the furnace again until it has been checked.
How a gas furnace starts, and where it stops
A gas furnace lights in a fixed order, and the step where it quits shows where to look:
- The thermostat calls for heat.
- The inducer motor starts, and a pressure switch confirms the vent is drawing.
- The hot surface igniter glows (older furnaces use a spark or a pilot flame).
- The gas valve opens and the burners light.
- The flame sensor proves the flame within seconds. If it cannot, the board shuts off the gas, retries, then locks out.
- The blower starts and pushes heat through the ducts.
Most control boards flash a fault code through a small window in the blower door. Note it before you cut the power; some boards lose it.
Electric furnaces
An electric furnace heats with resistance elements switched on in stages by sequencers or relays. Weak heat usually means one element or sequencer is out; no heat at all often traces back to a tripped breaker. These units draw heavy current, so we check the wiring connections for heat damage. Gas or electric, every visit ends with a safety check.
Common symptoms and what they usually mean
| What you notice | Likely cause | What it usually takes |
|---|---|---|
| Blower runs, air stays cold | Igniter, gas valve or a locked-out board | Ignition test, igniter or valve replacement |
| Burners light, then go out after a few seconds | Dirty or worn flame sensor | Sensor cleaning or replacement |
| Inducer runs, burners never light | Pressure switch, blocked vent or clogged condensate trap | Draft test, trap cleaning or switch replacement |
| Heats a few minutes, then stops while the blower keeps going | High limit tripping on poor airflow | Filter, return and blower checks |
| Pilot will not stay lit (older furnaces) | Thermocouple or dirty pilot | Thermocouple or pilot service |
| Loud boom when the burners light | Delayed ignition from dirty burners | Burner cleaning and gas pressure check |
| Squealing or grinding | Blower or inducer motor bearings | Motor replacement |
A burning-dust smell on the first run of the season is normal if it fades within an hour. A hot electrical smell is not: turn the furnace off and call. For more on no-heat calls see furnace not heating and furnace not igniting; for a furnace that turns on and off every few minutes, see furnace short cycling.
Attic, garage or closet: where it sits matters
- Attic furnaces lie on their side and are reached through the ceiling hatch, so a clear path to it saves time. On high-efficiency models the condensate runs through a trap and drain line; a water stain on the ceiling below is the early sign of a clog.
- Garage furnaces often stand on a platform. Keep gasoline, paint thinner and solvents away from them: the vapors can reach the burners.
- Hall-closet furnaces draw combustion air through a louvered door or vents. A closet packed with storage can starve the burners, which shows up as soot or a tripped rollout switch. Do not keep resetting a rollout switch; it trips for a reason.
Older furnaces in San Jose homes
In the 1950s–60s homes of Cambrian and Willow Glen, the furnace has often been replaced while the original ductwork stayed. Small returns and leaky joints make a newer furnace run hot, so a repeat limit trip there is more often an airflow problem than a bad switch. In Blossom Valley, some 1970s–80s furnaces still run on a standing pilot. A thermocouple is a small repair, but at that age the heat exchanger deserves the closer look. In Evergreen’s tract homes, builder-grade furnaces are reaching the age where a second or third repair in a few years makes it worth pricing a replacement too.
What affects the price
Most furnace repairs run $89–$600. Flame sensor cleaning, igniters and thermocouples sit at the low end; pressure switches, inducer and blower motors and control boards cost more, and attic access can add time. You get the price before work starts, and the diagnostic fee is waived if you go ahead with the repair. The furnace repair cost guide has more detail.
Repair or replace?
A single failed part on a sound furnace is worth fixing. A cracked heat exchanger, a failed combustion test or a string of repairs on a furnace past 20 years is when furnace replacement deserves a look. Our repair-or-replace guide walks through the numbers.
Before you call
- The thermostat is set to Heat, a few degrees above room temperature, with fresh batteries.
- The furnace power switch (it looks like a light switch near the unit) is on, and its breaker has not tripped.
- The filter is not clogged, and the blower door is seated: a loose door keeps a safety switch open.
A yearly furnace tune-up catches weak igniters and dirty sensors before the cold nights do.