How to Fix a Furnace Not Heating: Filter, Ignition, and Thermostat Diagnosis (2026)

Frequently Asked Questions

What is the igniter on a furnace and how do I know if it has failed?

Modern gas furnaces use an electronic igniter (hot surface igniter or HSI) instead of a standing pilot light. The igniter is a ceramic element that glows orange when the furnace tries to start. To check: with the furnace trying to start (thermostat calling for heat), open the furnace cabinet and look for the igniter glowing near the burner. If it glows but no flame follows: the gas valve may not be opening, or the flame sensor is dirty. If it doesn't glow at all: the igniter has failed. A hot surface igniter costs $20–$40 and is held by one or two screws — handle with gloves (skin oils shorten the lifespan). Replacement takes about 20 minutes.

What is a dirty flame sensor and how do I clean it?

The flame sensor is a rod-shaped probe that extends into the burner flame. It generates a small electrical current when surrounded by flame — this current tells the control board that ignition was successful. When the sensor is coated with oxidation (a film of white or light gray residue): it can't generate the current, and the furnace shuts down within a few seconds of lighting (a 'short-cycling' pattern — fires, runs briefly, shuts off, tries again). Clean the sensor with fine (0000) steel wool or an emery cloth — gently rub the metal rod once or twice and reinstall. This is the most common cause of a furnace that lights briefly then shuts off.

The furnace keeps cycling on and off (short cycling). What's causing it?

Short cycling (the furnace fires and shuts off repeatedly in a short cycle) has three main causes: (1) Dirty flame sensor — most common. Clean the sensor (see FAQ above). (2) Clogged air filter — the high-limit switch is tripping because the heat exchanger is overheating from restricted airflow. Replace the filter. (3) Cracked heat exchanger — the heat exchanger separates combustion gases from house air. A crack triggers the high-limit switch. This is a safety issue — a cracked heat exchanger requires professional diagnosis and usually furnace replacement.

What do furnace error codes (flashing LED lights) mean?

Most modern furnaces have a diagnostic LED (usually visible through the furnace cabinet window) that blinks in a coded pattern when a fault occurs. Common codes: 3 blinks = pressure switch failure (blocked condensate drain or flue obstruction); 4 blinks = high-limit switch open (overheating from clogged filter); 5 blinks = flame sensor failure; 6 blinks = ignition failure. Count the blinks in the pattern (blinks, pause, blinks = one complete code). Check the furnace manual (or find the label on the inside of the furnace cabinet door) for the code chart. Many codes direct you to a specific component.

My furnace won't turn on at all — no sounds, no light on the thermostat. What's wrong?

Complete loss of furnace function: (1) Check the circuit breaker labeled 'Furnace' or 'HVAC' in the panel — reset if tripped. (2) Check the furnace on/off switch (looks like a standard light switch, usually on the wall next to the furnace or at the top of the basement stairs). Confirm it's ON. (3) Some furnaces have a door safety switch — the furnace won't run if the cabinet panel is removed. Reinstall any removed panels. (4) If the thermostat is blank: the thermostat may have a dead battery (for battery-powered thermostats) or the C-wire (common wire) connection may be loose. Replace the thermostat battery or check the wiring.

Work through the checklist in order — most furnace problems are inexpensive DIY fixes.

What you need

• Replacement furnace filter (MERV 8–11, sized to your furnace — $10–$20)
• Hot surface igniter replacement ($20–$40 if igniter has failed)
• 0000 steel wool (for flame sensor cleaning)
• Screwdriver

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Step 1: Check the basics

Confirm: thermostat set to HEAT, setpoint above current temperature. Confirm: furnace switch is ON. Check the circuit breaker. If the thermostat display is blank: replace the batteries.

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Step 2: Replace the furnace filter

Locate the filter slot (usually at the air return duct connection at the furnace). Slide out the old filter. If it’s visibly dark and matted: replace it. A clogged filter causes overheating, which triggers the high-limit switch to shut the furnace down.

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Step 3: Reset the furnace

Turn the thermostat off. Turn the furnace power switch off. Wait 30 seconds. Turn the power back on. Turn the thermostat back to HEAT. Listen for the startup sequence: inducer fan runs, then igniter heats, then gas valve opens and burner lights. Watch through the cabinet window.

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Step 4: Check the igniter

If the furnace starts its sequence but no flame appears after 30–60 seconds: remove the access panel and look for the igniter element near the burners. If it doesn’t glow red/orange during the ignition attempt: it has failed. Replace per the FAQ instructions.

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Step 5: Clean the flame sensor

If the furnace lights briefly and then shuts off after a few seconds: the flame sensor is dirty. Locate the sensor (a thin rod extending into the burner area, held by one screw). Remove it. Lightly rub the metal rod with 0000 steel wool. Reinstall. Test.

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Related guides

• How to Change a Furnace Filter — proper filter replacement schedule and MERV guide
• How to Fix No Hot Water — hot water heater diagnosis (similar component structure)
• HVAC Maintenance Checklist — annual furnace maintenance to prevent failures