Your Furnace Won’t Light — Thermocouple/igniter faults vs. gas supply

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Few things are more frustrating than this:

“The furnace tries to start… but the flame never stays on.”

You might hear clicking.
You might see the blower run.
You might even see flame for a second—then nothing.

When a furnace won’t light, homeowners often jump straight to “bad furnace” or “no gas.” In reality, most no-light situations come down to a small number of overlooked issues, not total system failure.

80,000 BTU 80% AFUE Upflow/Horizontal Single Stage Goodman Gas Furnace - GR9S800803BN

Let’s walk through how techs actually diagnose this—and how you can tell what category your problem falls into.

🧠 First: How a Furnace Is Supposed to Light

Understanding the sequence matters.

A typical gas furnace lights in this order:

Thermostat calls for heat
Inducer motor starts
Pressure switch proves airflow
Igniter heats up (or pilot lights)
Gas valve opens
Flame lights
Flame is proven (sensor/thermocouple)
Blower starts

If any one step fails, the furnace shuts down for safety.

🔎 Big Picture: Three Categories of “Won’t Light”

When a furnace won’t light, the cause almost always falls into one of these:

🔥 Ignition failure (igniter or thermocouple)
🛢️ Gas supply problem
⚠️ Safety logic stopping ignition

The most overlooked issues live in category #3.

🔥 Igniter vs. Thermocouple — Know What Your Furnace Uses

Modern Furnaces: Hot Surface Igniters (HSI)

Glow orange/white hot
Light gas directly
Wear out over time (very common failure)

Older Furnaces: Standing Pilot + Thermocouple

Small pilot flame burns constantly
Thermocouple proves flame presence
If thermocouple fails → gas shuts off

📎 Furnace component basics:
👉 https://www.energy.gov/energysaver/furnaces-and-boilers

🧨 Overlooked Cause #1: Flame Sensor / Thermocouple Contamination

This is the most common reason a furnace lights briefly—then shuts off.

What Happens

Flame lights
Sensor doesn’t detect it
Gas valve closes within seconds
Furnace retries or locks out

Why It’s Overlooked

Flame is visible, so people assume it’s not the problem
The part looks fine
It’s tiny and easy to ignore

Reality

Oxidation coats the sensor and prevents it from reading flame properly.

This causes no-heat calls every winter.

📎 Manufacturer learning center:
👉 https://www.goodmanmfg.com/resources/hvac-learning-center

🛢️ Overlooked Cause #2: Gas Supply Is Present… But Not Enough

Many homeowners assume gas is either “on” or “off.”
That’s not how furnaces work.

Common Gas Supply Problems

Gas shutoff valve partially closed
Low inlet gas pressure during cold snaps
Regulator icing (rare, but real)
Other gas appliances drawing heavy load
Recently worked-on gas line with air trapped inside

What It Looks Like

Igniter glows
Gas valve opens
Flame is weak or uneven
Ignition fails or drops out

This gets misdiagnosed as an igniter failure all the time.

⚠️ Overlooked Cause #3: Pressure Switch & Venting Issues

This one stops ignition before gas ever flows.

What the Pressure Switch Does

It confirms that exhaust gases can safely leave the furnace.

If it doesn’t close:

Igniter may never energize
Gas valve never opens
Furnace appears “dead” during ignition

Why It’s Missed

No visible error
Furnace looks powered
Thermostat shows heat call
Homeowner focuses on gas instead of airflow

Common Winter Triggers

Frozen or blocked vent pipe
Snow covering exhaust/intake
Weak inducer motor
Long or poorly pitched vent runs

📎 Furnace venting basics:
👉 https://www.energy.gov/energysaver/furnaces-and-boilers

🔌 Igniter Faults (Still Common, Just Not Always the Culprit)

Igniters do fail—often quietly.

Signs of a Failed Igniter

No glow at all
Cracked ceramic
Furnace clicks but never lights
Repeated failed ignition attempts

Jake’s Reality Check

Igniters are wear items. If yours is 4–6 years old, it’s a prime suspect—but confirm before replacing.

🧯 Safety Lockouts That Prevent Lighting

Modern furnaces remember failures.

Lockout Triggers

Too many failed ignition attempts
Overheating events
Flame sensed when it shouldn’t be
Pressure switch faults

Once locked out, the furnace will not light again until reset.

🧠 Homeowner Diagnostic Flow (Safe Steps Only)

✅ Step 1: Observe

Does inducer motor run?
Does igniter glow?
Does flame appear briefly?
Any blinking error codes?

✅ Step 2: Check the Obvious

Gas shutoff valve fully open
Furnace power on
Air filter clean
Vents clear (inside and outside)

✅ Step 3: Reset Once (Not Repeatedly)

Thermostat OFF
Furnace power OFF for 2 minutes
Restore power and call for heat

Repeated resets without fixing the cause can make things worse.

🛑 When to Stop DIY Immediately

Call a professional if:

You smell gas
Flame lights violently or bangs
Furnace locks out repeatedly
You’re unsure about gas or electrical components
CO detector alarms

📎 CO safety reference:
👉 https://www.cdc.gov/carbon-monoxide/about/index.html

🧠 Jake’s Quick Comparison: What’s Most Likely?

Symptom	Likely Cause
Igniter glows, no flame	Gas supply issue
Flame lights then shuts off	Flame sensor / thermocouple
No glow, no flame	Pressure switch / igniter
Works after reset, fails again	Safety lockout
Only fails in cold weather	Venting / pressure switch