A furnace that tries to start but won’t ignite has a break somewhere in its lighting chain: no spark or glow, no gas reaching the burners, or a flame that lights but isn’t detected, so the control shuts the gas off again. The usual suspects are a dirty flame sensor, a failed igniter, a gas valve problem, a worn thermocouple on older furnaces and, less often, the control board. None of these is a part a homeowner should replace, but a few minutes of watching narrows it down.
Gas smell and no flame is an emergency. Unburned gas may be collecting. Leave the house without touching any switch or the thermostat, don’t try to relight anything, and call the gas company or 911 once you’re outside. If a carbon monoxide alarm is sounding, get everyone out first.
Three kinds of ignition
Look through the window or louvers in the furnace door during a start attempt.
- Hot surface igniter. A small element glows bright orange for several seconds, then the burners light. This is common on modern gas furnaces. The element is brittle and wears thin with use.
- Intermittent pilot. You hear rapid ticking as a spark lights a small pilot flame, which then lights the main burners.
- Standing pilot. Older furnaces keep a small flame burning all the time. A thermocouple sitting in that flame tells the gas valve it’s safe to stay open. If the pilot blows out or the thermocouple is worn, nothing lights.
Where the start attempt stops
On a modern furnace the order never changes. A small draft fan, the inducer, spins up; a pressure switch proves the venting is open; the igniter heats or sparks; the gas valve opens; and the flame sensor gets a few seconds to confirm a flame. The step where it quits tells you most of what you need:
- The inducer hums, but no glow or spark ever comes. The pressure switch hasn’t closed (a blocked vent, a clogged condensate drain, a weak inducer), or the board isn’t powering the igniter.
- The igniter glows, no flame. The gas valve isn’t opening, the manual valve is shut, or the igniter is too weak to light the gas.
- Flame for a moment, then out, then another try. The flame sensor can’t prove the flame, most often because it’s coated with residue.
- Several tries, then silence. The board has locked out after repeated failures. Many boards try again on their own after a waiting period.
Ignition faults at a glance
| Symptom | Likely cause | What it takes |
|---|---|---|
| Clicks, no glow, no flame | Failed igniter, open pressure switch or a board fault | Igniter test and replacement; switch and vent check |
| Bright glow, burners never catch | Closed or failed gas valve, low gas pressure | Valve test; gas pressure check |
| Flame appears, gone in seconds | Dirty or worn flame sensor | Professional sensor cleaning or a new sensor |
| Standing pilot won’t stay lit | Worn thermocouple or a dirty pilot | Thermocouple adjustment or replacement |
| Ticking spark, no pilot flame | Spark electrode, pilot orifice or gas valve | Pilot assembly service |
| Loud bang as the burners light | Delayed ignition | Burner and ignition service before further use |
| No attempt at all, blinking code | Control board or a safety switch | Board diagnosis and replacement if it has failed |
What you can safely do
- Make sure the thermostat is calling for heat: Heat mode, set well above the room temperature.
- Check the furnace’s power switch and its breaker.
- If the furnace vents through two white plastic pipes on an outside wall, look for leaves, a nest or a toy stuffed into the ends. Don’t climb onto the roof.
- Count the flashes of the status light and write them down; the label inside the door explains them.
- Try one reset: power off for a minute, then back on. If it fails the same way, stop there.
- If you find the gas valve handle turned across the pipe and don’t know why, leave it that way and call.
Leave these to a technician
The igniter, flame sensor, thermocouple, gas valve and control board all sit in or beside the burner compartment. Hot surface igniters crack if bumped, gas valves need a pressure check after any work, and a misread control board is an expensive guess. A technician measures the igniter’s resistance, the flame sensor’s microamp signal and the gas pressure before replacing parts. Never hold a gas valve open by hand, bypass a safety switch or keep resetting a furnace that keeps locking out. If a bang on ignition or soot raises the question of a cracked heat exchanger, the furnace is not restarted until it has been inspected.
What it costs
Most furnace repairs fall between $89 and $600; a control board or gas valve can run higher. A flame sensor cleaning or a new igniter usually sits near the lower end. You approve the price before the work, and the diagnostic fee is waived if you go ahead. On a cold night, our emergency HVAC service takes urgent calls 24/7.
Why it happens on the first cold night
San Jose winters are mild, so many furnaces sit idle from spring until the first chilly evening in late fall. Dust settles on the burners and the flame sensor over those months, and an igniter that was near the end of its life tends to give out within the first few starts. Running the heat for a test in October, or booking a furnace tune-up, moves that failure to a day when you don’t need the heat. If the furnace lights but the house still won’t warm, see furnace not heating.