Why is my furnace not igniting?

The most common reasons a furnace won't ignite are: (1) a cracked or burned-out hot surface ignitor, (2) a dirty or failed flame sensor that shuts off the gas before a flame can establish, (3) a gas supply problem such as a closed shutoff valve or low gas pressure, (4) a pressure switch stuck open or a blocked drain/flue that prevents it from closing, and (5) a control board fault or locked-out error code. Check the ignitor first — it's the most common single-point failure on modern gas furnaces.

Why does my furnace click but not ignite?

If your furnace clicks but won't ignite, the ignition sequence is starting but failing before flame establishment. Most common causes: the hot surface ignitor is cracked or burned out (no glow), the gas valve isn't opening (valve fault or no call for gas), or the flame sensor is too dirty to detect the flame and cuts the gas within a few seconds of ignition. Listen for the ignitor glow (a faint orange visible through the sight glass) and whether you hear the gas valve click open.

How do I know if my furnace ignitor is bad?

A bad furnace ignitor shows one of two failure modes: visually cracked or broken (you can often see a hairline crack in the silicon nitride or silicon carbide element), or it glows orange but not bright enough to ignite gas. Test it with a multimeter on resistance — most hot surface ignitors read 40–90 ohms when functional. An open circuit (infinite resistance) means it's failed. Ignitor replacement is typically under 15 minutes and parts cost $20–60 depending on the manufacturer (Carrier, Lennox, Trane, Rheem, Goodman all use proprietary form factors).

What causes a furnace to light then immediately shut off?

A furnace that lights then shuts off within 2–5 seconds almost always has a dirty flame sensor. The flame sensor is a metal rod that detects the burner flame via electrical conductivity — if it's coated with oxidation, it can't confirm the flame is present and the control board shuts off the gas as a safety measure. Clean the sensor rod with fine steel wool or an emery cloth. The fix takes under 10 minutes and costs nothing. If the problem persists after cleaning, the sensor itself may need replacement ($15–30).

Why is my gas furnace not turning on at all?

If the furnace doesn't respond at all — no sounds, no blower, no ignition sequence — check these first: (1) thermostat is calling for heat and set above current room temp, (2) furnace switch (looks like a light switch, often on the unit or nearby wall) is ON, (3) circuit breaker hasn't tripped, (4) furnace door panel is fully closed and latched (interlock switch), (5) the control board isn't displaying a fault/lockout code. A furnace in hard lockout after three failed ignition attempts won't retry until power is cycled.

Furnace Not Igniting? A Field Tech's Step-by-Step Diagnostic Flow

Quick Answer

A furnace that won't ignite usually fails for one of five reasons: bad hot surface ignitor, dirty flame sensor, gas supply problem, pressure switch issue, or a locked-out control board. Start with the ignitor — it's the most common single-part failure on modern gas furnaces and takes under 15 minutes to swap.

You get the call. Tenant says the heat's been off since last night and it's 58 degrees inside. The thermostat is calling for heat, the furnace is trying — you can hear the inducer spin up — but nothing's lighting. Before you pull the unit apart, run through this sequence. Each step eliminates a failure mode in order of likelihood.

The 5 Most Common Causes of a Furnace Not Igniting

Failed hot surface ignitor (HSI)

The most common cause on any furnace made after the mid-1990s. The silicon nitride or silicon carbide element cracks with thermal cycling and simply stops glowing. No glow = no ignition. Typical fix time: under 15 minutes.

Dirty or failed flame sensor

Furnace lights, burns for 2–5 seconds, then shuts off. The flame sensor rod is coated with oxidation and can't confirm flame presence. Control board cuts the gas as a safety response. A 10-minute clean fixes it most of the time.

Gas supply issue

Shutoff valve closed, low gas pressure from the utility, or a gas valve that's failed and won't open. You'll hear the ignitor glow (or click on a spark ignition system) but no flame establishes at all.

Pressure switch open / blocked drain or flue

The pressure switch verifies the inducer is creating proper draft before allowing ignition. A blocked condensate drain, blocked flue, or failed inducer motor keeps it open. The furnace won't even begin the ignition sequence if the pressure switch is open.

Control board fault / lockout

After three failed ignition attempts, most boards enter hard lockout and won't retry until power is cycled. The board may also have failed entirely. Check the LED fault code on the board — most manufacturers print the code chart on the furnace door panel.

The Diagnostic Flow — Work Through These in Order

Don't jump to parts replacement. This sequence moves from cheap and common to expensive and rare.

Read the Fault Code

Before touching anything, look at the control board LED. Most modern gas furnaces (Carrier, Lennox, Trane, Rheem, Goodman) have a diagnostic LED that flashes a code when there's a fault. The code chart is almost always printed on the inside of the furnace door panel.

Common codes and what they mean:

3 flashes (most brands): Pressure switch open — inducer isn't creating adequate draft
4 flashes: High-limit trip — overheating, check airflow and filter
5 flashes: Flame sensor / flame not detected after ignition
6 flashes: Ignition failure (no flame established after multiple attempts)
Continuous on: Board in hard lockout — cycle power to reset

Power cycle before diagnosing: If the board is in hard lockout, shut the furnace switch off for 30 seconds, then back on. This resets the lockout counter and lets you watch the ignition sequence from the beginning.

Inspect the Hot Surface Ignitor

The hot surface ignitor is a fragile ceramic element that glows orange-white at 1,800°F to ignite the gas. They break down with age and thermal stress — on average, they last 3–7 years. This is the first physical component to check when the fault code points to ignition failure.

To inspect:

Shut off power at the furnace switch
Locate the ignitor — it sits in the burner assembly, usually near the gas manifold
Look for a visible crack in the element — often a hairline fracture that's hard to spot
Test with a multimeter set to resistance: a functional ignitor reads 40–90Ω. Open circuit (infinite resistance) = failed

If failed, replace it. Ignitors are brand-specific in terms of form factor but readily available. Carrier uses part 62-24140-01 (and similar), Lennox uses H6024-16, Rheem/Ruud use 62-23543-01. Always verify part number from the unit's model tag.

Typical fix time: under 15 minutes. Two screws, one connector plug. Never touch the ceramic element with bare hands — skin oils reduce ignitor lifespan significantly. Handle by the bracket only.

Clean the Flame Sensor

If the furnace lights briefly — you see a flame for 2–5 seconds before it shuts down — the flame sensor is the first thing to clean. The sensor is a metal rod (usually stainless steel) that sticks into the burner flame. It passes a small electrical current through the flame to confirm it's present. When it's coated with oxidation, it can't complete the circuit and the control board shuts off the gas.

To clean:

Shut off power at the furnace switch
Locate the flame sensor — single rod with one wire connector, positioned so the tip extends into the flame path
Remove the mounting screw (usually 1/4” hex head)
Lightly scrub the metal rod with fine steel wool or an emery cloth — 10–15 strokes is enough. You're removing the oxidation layer, not polishing it.
Reinstall, restore power, and test

Typical fix time: under 10 minutes. Cost: $0. If cleaning doesn't fix it and the sensor tests good (0.5–10 microamps of signal during operation), the issue is elsewhere — likely gas valve or pressure switch.

Check the Pressure Switch and Inducer

The pressure switch is a safety that verifies the draft inducer is creating adequate negative pressure before the ignition sequence starts. If it stays open, the furnace won't attempt ignition — the sequence stops after the inducer run.

Check in this order:

Is the inducer running? You should hear it spin up at the start of the cycle. If it's not running, suspect inducer motor failure or a wiring issue.
Is the condensate drain clear? On 90%+ efficiency (condensing) furnaces, a blocked condensate drain causes water to back up into the pressure switch hose, keeping the switch open. Check the drain line and trap for blockage.
Is the flue clear? A bird nest, ice dam, or debris in the exhaust pipe blocks draft and keeps the pressure switch open. Check the pipe termination at the exterior wall.
Test the switch directly: Use a multimeter on the pressure switch terminals — it should close (continuity) when the inducer is running. If it stays open with the inducer running and the drain/flue are clear, the switch itself has failed.

Manufacturer note: Trane and American Standard furnaces are known for pressure switch hose deterioration — the rubber hose from the inducer housing to the switch cracks and loses seal. Inspect the hose carefully before condemning the switch itself.

Verify Gas Supply and Gas Valve Operation

If the ignitor glows but no flame appears, gas isn't reaching the burners. Work through gas supply in order:

Manual shutoff valve: The valve on the gas supply pipe entering the furnace should be parallel to the pipe (open). Perpendicular = closed. It's a simple check that gets missed.
Other gas appliances: If the water heater or range is also out, you have a supply issue upstream — call the utility, not an HVAC tech.
Gas valve operation: With the furnace calling for heat and the ignitor glowing, the gas valve should click open at the correct point in the sequence. You should see a flame appear within 1–2 seconds of the ignitor reaching full glow. If the ignitor glows but no click and no flame — the valve may have failed.

Gas valve replacement is straightforward — shut off the gas, replace the valve assembly, test for leaks with soap solution — but it requires proper licensing in most jurisdictions. If you've confirmed gas supply is present and the valve isn't opening, this is the part to replace or escalate.

Gas leak protocol: If you smell gas at the furnace, do not power cycle or attempt ignition. Shut off the gas supply at the manual valve, ventilate the space, and call the gas utility. Do not troubleshoot a furnace with a suspected gas leak.

When to Escalate to a Licensed HVAC Tech

Stop and call in if:

You smell gas at any point during diagnosis
The gas valve is confirmed to not open with power applied and proper call for heat
The heat exchanger is cracked (visible cracks, soot deposits around the exchanger, CO alarm trips)
The control board is showing fault codes you can't match to any field-fixable cause
The inducer motor is failed and needs replacement on a warranty unit

Everything before that — ignitor swap, flame sensor clean, pressure switch inspection, condensate drain clearing — is maintenance-level work. Gas valve replacement and heat exchanger diagnosis is licensed HVAC territory.

FixAtlas

This is the same diagnostic flow FixAtlas walks you through — on every call.

FixAtlas gives maintenance techs and property managers structured troubleshooting workflows, manufacturer-backed data, and real-world repair guidance for HVAC, plumbing, electrical, and appliances. Free during early access.

Get Early Access — Free →