Furnace Ignitor and Pilot Light Problems Explained

A furnace that will not light is one of the most common no-heat problems in Chicago homes. You might hear the furnace start up, see a glow through the viewing window, and hear a few clicks, but the burners never stay lit and the house keeps getting colder. Most of the time the problem lies in the ignition system: the ignitor, the pilot assembly or the flame sensor.

This guide explains how each type of ignition works, the symptoms of each failure, what is safe for a homeowner to check, and what a technician does during furnace repair.

How Furnace Ignition Works

Every gas furnace needs two things to make heat safely: a way to light the gas, and a way to confirm the flame is actually there. If the furnace cannot confirm a flame, it shuts the gas off within seconds so unburned gas does not build up.

Over the decades, furnaces have used three main ignition methods:

• Standing pilot. A small flame burns continuously and lights the main burners when heat is needed. Common in furnaces from decades ago.
• Intermittent pilot (spark ignition). An electric spark lights a pilot only when heat is needed, and the pilot then lights the burners.
• Hot surface ignition. A ceramic or silicon-based element heats until it glows, and the gas lights when it touches it. This is the most common system in furnaces installed in recent decades.

Chicago has a wide mix of housing, from 1920s bungalows to new condo buildings, so all three types still turn up. Knowing which one you have helps you understand the symptoms.

How to Tell Which Ignition Type You Have

With the furnace off, look through the small viewing window in the furnace door, or check the label inside the door:

• If you see a small, steady blue flame burning even when the furnace is not heating, you have a standing pilot.
• If you hear a rapid ticking at start-up followed by a small flame, you have intermittent pilot spark ignition.
• If you see a flat or rod-shaped element glow bright orange for several seconds before the burners light, you have a hot surface ignitor.

The rating plate and the instructions label inside the door will usually say which system the furnace uses.

Hot Surface Ignitor Problems

A hot surface ignitor works like the filament in an old light bulb. When the furnace calls for heat, it sends current through the ignitor, which heats until it glows. The gas valve opens and the gas lights on contact.

Ignitors are a wear item. Each heating cycle heats and cools the element, and over time it can crack or weaken. Common symptoms of a failing hot surface ignitor include:

• The inducer motor runs, but the ignitor never glows.
• The ignitor glows dimly or only part of it glows.
• The furnace tries several times and then locks out.
• The control board flashes an ignition failure code.
• The furnace works sometimes and not others, especially after a long off period.

Hot surface ignitors are fragile. Touching the element with bare fingers can leave oils that create hot spots and shorten its life, and a light knock can crack it. Replacement is a common repair, and a technician will also check why it failed, because frequent short cycling or incorrect voltage can wear an ignitor out early.

What a Technician Checks

A technician will watch the start-up sequence, check that the ignitor is receiving power from the control board, and test the ignitor's resistance with a meter. If the ignitor is good but is not receiving power, the problem may be earlier in the sequence, such as the pressure switch or control board.

Flame Sensor Problems

Once the burners light, the furnace needs to prove the flame exists. Most modern furnaces use a flame sensor: a thin metal rod that sits in the flame. It carries a tiny electrical signal through the flame back to the control board. If the board sees that signal, it keeps the gas valve open. If not, it closes the valve.

Over time, the sensor rod gets coated with a thin layer of residue from combustion. That coating weakens the signal. The classic symptom is a furnace that:

• Lights normally,
• Runs for a few seconds,
• Then shuts the burners off,
• And repeats this several times before locking out.

This is one of the most common causes of furnace short cycling. A technician cleans the sensor carefully, checks its position in the flame, and measures the signal. If the sensor is cracked or the signal stays weak after cleaning, it is replaced. Grounding and wiring issues can also weaken the signal. For more on start-stop patterns, see our guide to furnace short cycling during a cold snap.

Standing Pilot Light Problems

Older furnaces with a standing pilot use a thermocouple or thermopile to prove the pilot is lit. The device sits in the pilot flame and generates a small voltage that holds the gas valve open. If the pilot goes out, the voltage drops and the gas valve closes.

Common pilot problems include:

• A pilot that will not stay lit. Often caused by a worn or misaligned thermocouple.
• A weak, yellow or flickering pilot flame. Usually caused by a dirty pilot orifice or poor combustion air. A healthy pilot is mostly blue.
• A pilot blown out by drafts. Basements in older Chicago homes can be drafty, and a strong draft can blow out a pilot.

Many standing-pilot furnaces have relighting instructions printed on a label inside the door, and some homeowners follow them. If you choose to relight a pilot, follow the label exactly. If the pilot will not stay lit after relighting, or you smell gas at any point, stop. Leave the area if you smell gas and call Peoples Gas at 866-556-6002 from outside.

A furnace old enough to have a standing pilot is likely well past its expected lifespan. If it is failing often, read our guide to how long a furnace lasts in Chicago before putting money into major repairs.

Intermittent Pilot and Spark Ignition Problems

Intermittent pilot systems use a spark electrode to light a small pilot, and a sensor then proves the pilot before the main burners open. Common failures include:

• A cracked or misaligned spark electrode that sparks but does not reach the gas.
• A worn ignition module that no longer sends a strong spark.
• A dirty pilot assembly that produces a weak flame the sensor cannot detect.
• Continuous ticking with no flame, which means the spark is firing but gas is not lighting.

Spark systems involve high voltage and gas, so diagnosis and repair should be left to a technician.

Problems That Look Like Ignition Failure but Are Not

Several other faults stop a furnace from lighting and can be mistaken for a bad ignitor:

• Pressure switch problems. The inducer motor must create enough draft to close the pressure switch before ignition starts. A blocked vent, a cracked hose, a failing inducer or a clogged condensate trap can keep the switch open.
• Gas supply problems. A closed manual valve, a failed gas valve or a supply issue means there is nothing to light.
• Limit switch problems. If the furnace overheated because of a dirty filter, a limit switch may prevent the burners from lighting.
• Control board failures. The board runs the entire start-up sequence. If it fails, nothing happens in the right order.
• Thermostat or wiring issues. If the call for heat never reaches the furnace, the ignition sequence never starts.

This is why a proper diagnosis matters. Replacing an ignitor will not fix a pressure switch problem, and guessing at parts gets expensive. Our furnace won't turn on checklist covers the safe checks to rule out simpler causes first.

What Homeowners Can Safely Check

You can safely:

• Confirm the thermostat is set to heat and has power.
• Check the furnace power switch and breaker.
• Replace the air filter.
• Make sure the blower door is closed securely.
• Check that outdoor PVC vent pipes on a high-efficiency furnace are not blocked by snow or ice.
• Note any error code flashing on the control board before resetting power.
• Watch the start-up sequence and note what happens: inducer, glow, click, flame, shutdown.

You should not handle a hot surface ignitor, adjust the gas valve, clean burners, bypass safety switches or work on the control board. Many guides online show homeowners cleaning flame sensors. It is a simple task for a technician, but it involves removing parts from the burner area of a gas appliance. If you are not trained, leave it to a professional.

Why Ignition Parts Fail Early

Some furnaces go through ignitors and flame sensors faster than others. Common reasons include:

• Short cycling. Every start is wear on the ignitor. Oversized furnaces or thermostat problems that cause frequent starts shorten ignitor life.
• Dirty combustion. Dirty burners and poor combustion air leave more residue on flame sensors.
• Incorrect voltage. Some ignitors are sensitive to supply voltage.
• Rough handling. Hot surface ignitors can crack when bumped during other service.
• Skipped maintenance. A yearly tune-up catches weak ignitors and dirty sensors before they fail.

Preventing Ignition Problems

An annual tune-up before heating season is the best defense. During a tune-up, a technician typically checks the ignitor, cleans and tests the flame sensor, inspects burners and pilot assemblies, checks gas pressure and venting, and tests safety controls. Catching a weak ignitor in September is much better than losing heat during a January cold snap. See our tune-up and maintenance page for details.

Repair or Replace?

Ignitors, flame sensors and thermocouples are relatively inexpensive parts, and replacing them usually makes sense even on an older furnace. The calculation changes if ignition problems come with other issues: a cracked heat exchanger, repeated control board failures, or a furnace that is 20 years old or older with a history of breakdowns. In those cases, compare the repair to a new system. Our page on furnace replacement in Chicago and our guide to what affects furnace replacement cost will help.

Get Your Furnace Lighting Again

If your furnace clicks, glows or tries to start but never makes heat, call (312) 638-0812. A technician can test the ignitor, flame sensor, pressure switch and control board, explain what failed and give you written options before any work begins. In very cold weather, see how emergency HVAC service handles no-heat calls across neighborhoods such as Lakeview, Portage Park and Edgewater.