A furnace that isn’t heating has usually been stopped by something small: a thermostat setting, a clogged filter, a switch that’s off, a pilot that has gone out, or a single worn ignition part. Less often the blower motor or the gas supply is to blame. Before you call, notice how the furnace behaves, because each pattern points somewhere different.
Gas smell? Leave right away. Don’t flip switches or touch the thermostat on the way out, and phone the gas company or 911 once you’re outside. CO alarm sounding? Everyone leaves the house, and you call 911 from outside.
When nothing happens at all
No blower, no clicks, no hum. The furnace is either without power or not receiving a call for heat. Start with the thermostat (Heat mode, set above room temperature, fresh batteries), then the breaker and the power switch mounted near the furnace. Most furnaces also have a safety switch behind the blower door: if the panel was removed for a filter change and not pushed fully home, the furnace won’t run. When all of that checks out, suspect a blown low-voltage fuse, a failed transformer or a bad control board.
When the fan blows cold air
Look at the thermostat’s fan setting first. In the On position the blower never rests, so between burner cycles it circulates unheated air; Auto is what you want. With the fan on Auto and the air still cold, the burners aren’t lighting or aren’t staying lit. On a modern furnace that points to the igniter, the gas valve, a pressure switch or a dirty flame sensor, and our page on a furnace that won’t ignite walks through each. Older furnaces that keep a small pilot flame burning may simply have lost it.
When it heats, but not enough
The burners light and warm air comes out, yet the house won’t warm up or some rooms lag far behind. Typical reasons: a clogged filter that lets the furnace overheat and shut its burners off early, a weak blower motor, closed or blocked registers, and leaky or disconnected ducts dumping heat into the attic or crawlspace. If the burners keep stopping and restarting every few minutes, see furnace short cycling.
When it tries and gives up
You hear the furnace start, perhaps a click or an orange glow, then it stops and tries again. After a few failed attempts most furnaces lock out for a while. A flashing LED behind the small window in the furnace door usually gives a code; note the pattern before you cut power, because switching off clears it.
At a glance
| What you notice | Likely cause | What it takes |
|---|---|---|
| Blower runs all day, air never warms | Thermostat fan left on On | Return the fan to Auto |
| Dead silent, no lights | Power off, blower door switch open, board fuse or transformer | Switch or breaker; electrical diagnosis |
| Igniter glows orange, burners stay dark | Gas shut off at the valve, valve failure, supply outage | Valve check; testing or a utility call |
| Burners catch, then drop out within seconds | Flame sensor coated or worn | Clean or replace the sensor |
| Pilot goes out or won’t relight | Thermocouple or pilot assembly | Thermocouple replacement |
| Warm air, very little of it | Clogged filter, failing blower motor | New filter; motor or capacitor repair |
| Hot air at the furnace, cool rooms down the hall | Duct joints separated or leaking | Duct sealing or reconnection |
Safe checks for you
- Thermostat: Heat mode, three or four degrees warmer than the room, fan on Auto, weak batteries replaced.
- Filter: if it’s grey or caked, put in a new one. Standard filters typically need it every one to three months.
- Power: the breaker, and the furnace’s own power switch.
- Blower door: seated firmly so its safety switch closes.
- Gas: the handle on the gas valve beside the furnace should run parallel to the pipe. If the stove and water heater have no gas either, the supply is out; call the gas company.
- Pilot (older furnaces only): relight it with the instructions printed on the furnace, and only if there’s no gas smell.
- Registers and returns: open and unblocked.
Stop and call when
- the furnace still won’t heat after those checks, or locks itself out a second time;
- the pilot won’t stay lit, or the burners light with a bang;
- you see black soot marks near the furnace or a lazy yellow flame;
- a technician has told you the heat exchanger may be cracked. Then the furnace stays off: don’t restart it, because a crack can leak carbon monoxide into the house.
Anything inside the burner compartment is technician work: the igniter, flame sensor, gas valve, control board and blower motor. Our technician runs the furnace through its start-up, measures gas pressure and the flame signal, checks airflow, and inspects the flue and heat exchanger before quoting a price.
Furnace repair costs
Most furnace repairs fall between $89 and $600, with larger parts running higher. Flame sensor cleaning, igniters and thermocouples sit toward the lower end; blower motors, gas valves and control boards cost more. The diagnostic fee is waived if you go ahead. A yearly furnace tune-up, $89 to $199, is the usual way to catch a dirty sensor or a tired igniter before the first cold night.
Older San Jose ducts
Many older San Jose homes, including plenty in Cambrian and Willow Glen, still have original ductwork in the attic or crawlspace. When the furnace runs normally but the house stays cool, a separated or leaking duct is a common reason, and a new furnace wouldn’t fix it.