Furnace Won't Ignite — No Heat When Thermostat Calls

When the thermostat clicks on but the furnace never fires, the problem is usually the igniter, flame sensor, or gas valve. Start with the simplest fix.

Category:HVAC

Difficulty:Moderate

Time:30-45 min

Success:50%

Updated:May 22, 2026

quick_referenceQuick Answer

For Furnace Won't Ignite — No Heat When Thermostat Calls, start with "Watch the startup sequence through the sight glass": Remove the furnace front panel and find the small round sight glass on the burner compartment. Set the thermostat to call for heat, then watch what happens through the glass. You should see: inducer fan spin up, pressure switch click, igniter glow bright orange for 5-10 seconds, gas valve open with a click, burners light, and flame sensor keep everything running. Whatever step it stops at — that's your failure point. Count the LED flashes on the control board too, the blink code is taped to the inside of the panel. Stop DIY if you smell gas anywhere near the furnace or gas line connections. This is listed as a moderate recovery and usually takes about 30-45 min.

verifiedGuide Snapshot

Repair areaHVAC

Estimated time30-45 min

DifficultyModerate

Stop conditions4

Last updated May 22, 2026. Review the stop conditions before continuing.

account_treeRecovery State

DeviceFurnace

Current stateNot Igniting

Specific stateNo Heat On Thermostat Call

Failed stepFurnace Startup Sequence

Likely failure typeElectrical Fault

DIY boundaryDIY recovery first

paymentsCost decision

help1. Understand the Problem

A furnace that won't ignite usually fails at one of three points in the startup sequence. The draft inducer fan spins up first — if it doesn't, the pressure switch never closes and the board won't proceed. Next, the hot surface igniter glows — if it's cracked or burned out, the gas valve won't open. Finally, the flame sensor proves the flame — if it's dirty, the system lights then immediately shuts off. Each failure point has a distinct set of symptoms.

build_circle2. Try This First

Best First Step

Watch the startup sequence through the sight glass

Remove the furnace front panel and find the small round sight glass on the burner compartment. Set the thermostat to call for heat, then watch what happens through the glass. You should see: inducer fan spin up, pressure switch click, igniter glow bright orange for 5-10 seconds, gas valve open with a click, burners light, and flame sensor keep everything running. Whatever step it stops at — that's your failure point. Count the LED flashes on the control board too, the blink code is taped to the inside of the panel.

visibility3. Visual Guidance

See what's happening and how to try the first recovery step.

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Check the igniter for cracks or burnoutThe hot surface igniter is a flat gray or black ceramic piece about 2 inches long near the first burner. If it's cracked, chipped, or has a white burnt spot in the middle, it's dead. Measure resistance with a multimeter — a good one reads 40-90 ohms. An open circuit means it's burned out. Replacement igniters are universal and cost about $25-40.

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Clean the flame sensor with steel woolIf the burners ignite for 2-3 seconds then shut off, the flame sensor is dirty. It's a thin metal rod on the opposite end of the burner assembly from the igniter — it'll have a single wire going to it. Remove the single screw holding it, rub the rod with fine steel wool until it's shiny, and reinstall. A dirty flame sensor is the number one cause of short-cycling furnaces.

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Test the pressure switch and inducerIf the inducer fan spins but the igniter never glows, the pressure switch isn't closing. Check the small rubber tube connected to it — these crack or get blocked with condensate. Pull the tube off and blow through it to clear any water or debris. If the inducer fan is noisy, wobbling, or not spinning at all, the motor or capacitor has failed and needs replacement.

autorenew4. If That Doesn't Work

Try the next recovery options.

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Check the thermostat and wiringSometimes the problem isn't the furnace at all. Pull the thermostat off its base and jumper the R and W terminals together with a short piece of wire. If the furnace fires right up, your thermostat is bad. Check for loose wires at the furnace control board too.

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Replace the control board fuseMost furnace control boards have a small 3-amp or 5-amp automotive blade fuse. A power surge or a shorted thermostat wire can blow it. Pull the fuse and check it with a multimeter or just look for the burned link inside. Replace with the exact same amp rating.

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Frequently Asked Questions

My furnace ignites for a few seconds, then the flames go out. What's wrong?expand_more

Classic dirty flame sensor. The control board fires the burners, checks for flame signal, doesn't see it because the sensor is coated in oxidation, and shuts the gas off as a safety measure. Clean it with steel wool and it'll fire right up.

How much does a furnace igniter cost to replace?expand_more

A universal hot surface igniter costs $25-40 at any hardware store or HVAC supply house. The install takes 15 minutes with a screwdriver and a nut driver. A service call for the same thing runs $200-350.

Why does my furnace click but nothing happens?expand_more

The click is the relay on the control board trying to start the sequence. If nothing follows, check the inducer fan first — if it doesn't spin, the pressure switch never closes and the board aborts. Could also be a blown control board fuse.