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Furnace won't ignite? Nine checks, in the order a tech runs them
A furnace that won't light is usually one cheap part — or one clogged filter — away from running. Work these checks in order before you pay for a service call, and stop the moment anything smells like gas.
Guide by Zach Barbre · Denver HVAC expert
Updated October 2026 · Written from first-hand field experience in the north Denver metro
If you smell gas (rotten-egg odor), leave the house without flipping any switches, and from outside call 911, then Xcel Energy's gas emergency line at 1-800-895-2999. If your carbon monoxide alarm is sounding, get everyone outside and call 911. Don't run the checks below until the house is cleared. Colorado law (C.R.S. § 38-45-102) requires a working CO alarm within 15 feet of every bedroom in a home with fuel-burning appliances — if you don't have one, that's today's project after the heat is back.
First, what exactly is it doing?
"Won't ignite" covers four different failures, and each points somewhere different. Watch one full cycle from the thermostat call and match it:
- Nothing happens at all — no sounds, no fan. Power, thermostat, or the blower-door safety switch. Checks 1, 2, and 5.
- A small fan starts (the inducer), you hear clicking, but no flame — the ignitor isn't glowing, or gas isn't arriving. Checks 4 and 6.
- Flame lights, then dies after 2–3 seconds, and the furnace retries a few times before quitting — the classic dirty flame sensor. Check 7.
- It lights and runs, but shuts off before the house warms up — airflow or a safety limit. Check 3, and our short-cycling guide.
The checks, in the order a tech runs them
1. Thermostat: mode, temperature, batteries
Set it to HEAT, fan on AUTO, and the temperature at least 3–5 degrees above the current room temperature. If the display is dim or blank, replace the batteries — a dying thermostat battery set imitates a dead furnace every fall. If you recently switched to a smart thermostat and the furnace stopped working the same week, tell the tech; wiring mismatches are common.
2. Power: the furnace switch and the breaker
Most furnaces have a plain light-switch-style cutoff on or near the unit — it gets bumped off during summer storage more often than you'd think. Check it's ON, then check the breaker panel for a tripped furnace breaker. Reset a tripped breaker once. If it trips again, stop — that's an electrical fault, not a DIY project.
3. The filter
Pull the filter. If it's grey, matted, or you can't see light through it, the furnace can overheat and shut itself down on its high-limit switch — sometimes before it ever gets going properly. Replace it (they're $10–$20) and retry. Filters are a monthly check in a Colorado winter, not an annual one — dry air and long run times load them fast.
4. Gas supply to the furnace
Confirm the gas valve handle on the pipe at the furnace is parallel to the pipe (open). Then check another gas appliance — a stove burner or the water heater. If nothing gas-fired in the house works, it's a utility problem, not a furnace problem: call Xcel Energy at 1-800-895-4999. If only the furnace is out, keep going.
5. The blower door
The front panel of the furnace has a safety switch that kills the ignition sequence if the door isn't seated. If anyone's opened the furnace recently — a filter change counts — reseat the door firmly and retry.
6. Watch the ignition sequence
With the thermostat calling for heat, stand by the furnace and watch through the sight glass: you should see the small inducer fan start, then a bright orange glow (the hot-surface ignitor) within a minute or so, then flame. No glow at all most often means a cracked ignitor — they last about 5–7 years and fail without warning. That's a pro replacement: ignitors are fragile ceramic, run on line voltage, and manufacturers spec them as a qualified-servicer part. Per our cost guide, an ignitor or flame-sensor repair runs $200–$400 installed.
7. Lights-then-dies: clean the flame sensor
If the burners light and cut out within seconds, the flame sensor — a thin metal rod in the flame's path — is usually coated with oxidation and can't prove flame to the control board, so the board shuts the gas off. Handy homeowners do clean these: power OFF at the switch, remove the single screw holding the sensor, wipe the rod firmly with a dollar bill or fine steel wool until it's bright metal, reinstall, power on. If it still drops out, the sensor or the board needs a tech.
8. High-efficiency furnaces: check the condensate and the vents
If your furnace vents through white PVC pipe (a 90%+ AFUE condensing furnace), it makes water as it runs — and a clogged condensate trap or drain line stops it cold, because a safety pressure switch won't let it fire with a flooded trap. Check the drain line for kinks and the trap for sludge. Then walk outside and check the PVC intake and exhaust pipes: drifting snow, a bird nest, or a sprinkler-soaked screen over the intake will shut a furnace down in a Colorado winter, and this is the single most missed no-heat cause we see.
9. Read the error code — then write it down
The control board flashes an LED pattern (for example, 3 blinks, pause, repeat). The code chart is on the inside of the blower door. Note the pattern before you reset anything — repeated resets can clear the evidence and lock the board out after failed retries. That blink code is the first thing a tech will ask about, and having it can shorten the visit.
What it costs if a part has failed
| Common no-heat repair | Typical range | Notes |
|---|---|---|
| Service / diagnostic visit | $75–$150 | Often applied to the repair if you approve it on the spot |
| Ignitor or flame sensor | $200–$400 | The most common no-heat fix; cheap part, mostly labor |
| Control board | $300–$800 | Model-specific; older boards can be hard to source |
| Blower motor | $400–$900 | ECM variable-speed motors cost far more than fixed-speed |
Full installed-price ranges for replacement, if it comes to that, are in the Denver-metro cost guide.
Stop and call instead if…
- You smell gas at any point, or your CO alarm sounds — that's the safety box at the top, not this list.
- You see scorch marks, melted wire insulation, or a yellow, lazy, rolling flame — signs of a combustion problem, up to a cracked heat exchanger.
- The breaker trips a second time, or the furnace locks out again after these checks. One reset is diagnosis; three is ignoring the diagnosis.
Questions homeowners ask about this
Why does my furnace click but not ignite?
Clicking is the control board trying — usually the ignitor relay or the gas valve. If you don't see an orange glow through the sight glass within about a minute of the inducer fan starting, the hot-surface ignitor has most likely failed. It's a $200–$400 professional repair, and it fails on a 5–7 year cycle, so a furnace on its second ignitor is telling you its age.
Can I replace a furnace ignitor myself?
The part is cheap ($15–$100) and the swap looks simple, but ignitors are brittle ceramic that shatter from skin oils or a small drop, they run on line voltage, and a mismatched voltage rating (80V vs 120V) can damage the control board. Manufacturers spec ignitor replacement for qualified servicers. The flame sensor cleaning in this guide is the DIY-friendly version of the same call.
My furnace runs but the house won't reach temperature — is that ignition too?
Not usually. If it lights and runs long cycles but can't keep up, think airflow (filter, blocked returns), a derating issue, or simple undersizing. At metro Denver's roughly 5,000 feet, manufacturers derate furnace input about 4% per 1,000 feet of elevation — a furnace sized off sea-level rules of thumb will run forever and never quite get there. A Manual J load calculation is how you find out which problem you have.
How long should I try before calling a tech?
Work the nine checks in order — most take under 20 minutes total. If the furnace still won't stay lit, call with the error-code blink pattern, the furnace's age, and what you observed in the ignition sequence. That information genuinely shortens the repair visit.
Rather have a pro just handle it?
We connect Denver-metro homeowners with licensed local installers — free second opinions, written quotes, no pressure. Tell them what you found in the checks above and you'll skip half the diagnosis.
Typical response time: under 15 minutes, 7am–7pm daily.