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Before opening the furnace cabinet or burner compartment, a technician will kill the breaker at the disconnect to de-energize the system — this is not a homeowner step. Refrigerant work is also federally regulated under EPA Section 608; certification is legally required.
A furnace that won’t ignite can point to a failed igniter ($100–$300 to replace), a dirty or failed flame sensor ($75–$250), or a faulty pressure switch. A cracked heat exchanger is the most serious failure: repair runs $600–$4,000 and can approach the price of a new gas furnace ($2,500–$6,500 installed), per our cost guide. If you smell gas, leave the house immediately without touching switches or gas valves, and call 911 or your gas utility’s emergency line from outside.
It's a January night, the temperature outside has dropped to 18°F, and your thermostat clicks on — but the furnace never fires. The air coming from the vents is cold. This is one of the most stressful HVAC emergencies a homeowner faces, combining immediate physical discomfort with real safety risk: a home without heat in winter threatens occupants, pets, and the plumbing in your walls.
Understanding what's actually failing — and why almost every cause requires a qualified professional rather than a DIY attempt — helps you respond quickly, communicate clearly with a technician, and avoid actions that make the situation worse or more dangerous. This article is part of our complete furnace diagnostic guide, which covers every furnace symptom, not just ignition.
How Does a Furnace Ignition System Work?
Modern gas furnaces use a multi-step ignition sequence controlled by the furnace's circuit board. Every step must complete successfully before the next begins. When any step fails, the board detects the fault, records an error code, and shuts down safely rather than allowing gas to accumulate.
- Thermostat call for heat: The thermostat sends a 24-volt signal to the furnace control board requesting heat.
- Draft inducer motor pre-purge: The inducer motor starts and pulls combustion gases out of the heat exchanger to clear it before gas is introduced. A pressure switch monitors that this airflow is actually happening — if the switch does not close, the sequence stops here.
- Igniter energizes: The hot surface igniter (a fragile silicon carbide or silicon nitride rod) heats until it glows hot enough to light the gas, or a spark igniter fires. The board monitors that the igniter is drawing the expected current — a failed igniter draws nothing.
- Gas valve opens: Once the igniter has reached ignition temperature, the gas valve opens and allows fuel to flow to the burners. This step happens in a very tight window — the valve opens only while the igniter is verified hot.
- Burner flame established: The igniter lights the gas. The resulting flame must be established within seconds, or the gas valve closes again as a safety measure.
- Flame sensor proves the flame: A metal rod in the burner flame passes a tiny electrical current through the flame. Flame conducts electricity; no flame means no current, and the board shuts the gas valve. This is the primary safety mechanism preventing gas from flowing without combustion.
- Blower motor starts: Once the heat exchanger reaches operating temperature (verified by a limit switch), the blower motor starts and pushes heated air through your ductwork.
If any step fails, the furnace shuts down and retries a limited number of times (it varies by manufacturer) before entering lockout mode — indicated by a blinking LED error code on the control board. Each blink pattern corresponds to a specific fault code documented in your furnace manual.
What Causes a Furnace Not to Ignite?
1. Failed Hot Surface Igniter — $100–$300
The hot surface igniter is a wear part: a fragile ceramic element that becomes brittle after years of heating and cooling. Igniters typically fail in one of two ways: a complete crack that prevents any resistance heating, or gradual weakening that makes the element too cool to reliably ignite gas.
What it sounds like: The inducer motor runs (a whooshing sound), there may be a brief click, but no combustion sound follows. The furnace repeats this a few times, then goes silent with a blinking error light.
Igniter problems often show up on the first cold night of the heating season, when the furnace runs after months of sitting idle. On our cost guide it is among the less expensive furnace repairs.
2. Dirty or Failed Flame Sensor — $75–$250
The flame sensor is a small metal rod positioned in the burner flame. Over time, a thin layer of oxidation builds up on the rod's surface, insulating it from the electrical signal the flame conducts. The result: the furnace successfully ignites, burns for a few seconds, then shuts off — because the dirty sensor cannot confirm that flame is present.
What it sounds like: The furnace ignites briefly — you may feel a small pulse of warm air — then shuts off. It repeats this cycle a few times before lockout.
A technician can often clean the flame sensor rather than replace it. Per our cost guide, flame sensor cleaning or replacement runs $75–$250, at the low end of furnace repair costs.
3. Faulty Pressure Switch
The pressure switch is a small diaphragm-operated device that verifies the draft inducer motor is pulling adequate airflow through the heat exchanger before allowing gas to flow. If the switch fails to close — due to a cracked diaphragm, a blocked condensate line, or a failing inducer motor — the control board interprets this as a ventilation failure and halts the ignition sequence at step 2.
What it sounds like: The inducer motor runs, but the furnace never proceeds to ignition. The error code is usually a pressure switch fault.
Pressure switch issues can sometimes stem from a blocked condensate drain line (common in high-efficiency condensing furnaces) rather than the switch itself — which is why professional diagnosis is valuable here before replacing parts. Our cost guide does not list a separate range for pressure switches, so ask for a written quote after diagnosis.
4. Gas Valve Failure — $200–$600
The gas valve is the electromechanical component that controls fuel flow from your home's gas supply to the furnace burners. When it fails, no gas reaches the igniter — and no combustion occurs regardless of how perfectly every other component functions. Gas valve failures can be either mechanical (the valve physically cannot open) or electrical (the solenoid that operates it has failed).
Gas valve diagnosis and replacement involves working directly with the gas supply line to your furnace. This must be performed by a qualified professional. Incorrect work can result in gas leaks, carbon monoxide exposure, or fire. Leave the gas valve, gas line, and burner compartment to the technician.
5. Cracked Heat Exchanger — $600–$4,000 repair or full replacement
The heat exchanger is the most critical safety component in a gas furnace. It is a set of metal chambers through which combustion gases travel on one side while your breathable air passes over the other. The exchanger transfers heat without allowing combustion gases — including carbon monoxide — to mix with the air circulated through your home.
A heat exchanger can crack from metal fatigue after years of heating and cooling, or from overheating. Some cracks disturb the draft or flame enough to trip the furnace's safety switches and stop ignition; others cause no obvious symptom, which is why a technician's inspection matters.
Signs beyond ignition failure: Yellow or flickering burner flame, soot around the furnace, a faint chemical smell when the furnace runs, or household members experiencing headaches or nausea during heating cycles.
A cracked heat exchanger diagnosis should never be dismissed or delayed. Per our cost guide, the repair runs $600–$4,000, which can approach the price of a new gas furnace ($2,500–$6,500 installed). ENERGY STAR suggests considering replacement once a furnace is more than 15 years old. See our article on carbon monoxide and your HVAC system for a full discussion of this risk.
6. Control Board Failure — $300–$900
The control board is the electronic brain of the furnace — processing thermostat signals, sequencing the ignition steps, monitoring safety switches, and managing the blower. When it fails, symptoms can be erratic and mimic almost any other component failure, making diagnosis particularly challenging. A board may fail completely, preventing any operation, or partially, causing intermittent issues that are difficult to reproduce on demand.
Control board diagnosis requires proper test equipment and HVAC electrical knowledge. It is never appropriate for homeowner troubleshooting beyond the basic checks described later in this article.
7. Dirty or Failing Inducer Motor — $200–$700
The draft inducer motor creates the negative pressure that draws combustion gases through the heat exchanger and out the flue. If it is failing — turning slowly, overheating, or drawing insufficient current — the pressure switch will not close and the ignition sequence stops before gas ever flows. A failing inducer motor often announces itself with a loud grinding, rattling, or screeching noise before complete failure.
How Much Does Furnace Repair Cost?
The table below shows the furnace ranges from our cost guide. Actual costs vary by region, provider, and whether emergency service is needed. A diagnostic or service-call fee typically runs $65–$150; some providers credit it toward the repair if you book it with them, so ask when you call.
| Component | Repair / Service | Typical Cost | Time |
|---|---|---|---|
| Hot surface igniter | Replacement | $100–$300 | 1–2 hrs |
| Flame sensor | Cleaning or replacement | $75–$250 | 1 hr |
| Gas valve | Replacement | $200–$600 | 2–3 hrs |
| Inducer motor | Replacement | $200–$700 | 2–4 hrs |
| Control board | Replacement | $300–$900 | 2–3 hrs |
| Heat exchanger | Repair | $600–$4,000 | 4–8 hrs |
| Full furnace replacement | New unit + installation | $2,500–$6,500 | 1 day |
| Night / weekend / holiday premium | Added to the repair cost | +$100–$300 | — |
Independent providers may be available to assist you in your area.
Call Now — (844) 582-1795Disclosure: We are a referral service and may receive compensation for qualified calls. Calls may be routed to an independent provider network and may be recorded. Pricing and availability vary by provider and location.
What Can I Check Before Calling a Furnace Tech?
These checks take under five minutes, are safe for homeowners, and occasionally reveal simple issues that resolve without a service call. None of them involve opening the furnace cabinet or touching any gas components.
- Thermostat settings: Confirm the thermostat is set to HEAT and the target temperature is above the current room temperature. Check battery life — a thermostat with dead batteries will send no signal at all. For smart thermostats, check that the app shows the device as connected and online.
- Tripped breaker: If you notice the furnace's breaker has tripped, leave it off, set the thermostat to OFF, and call a technician. A breaker that trips is protecting against a fault the technician needs to find; do not reset breakers yourself.
- Furnace power switch: A standard-looking light switch is typically mounted on the wall near the furnace or on the unit itself, and it is easy to switch off by accident. If it looks off, mention that when you call rather than flipping it yourself; the technician can confirm it is safe to restore power.
- Air filter condition: A severely clogged filter restricts airflow past the heat exchanger, causing the limit switch to trip and shut the furnace down to prevent overheating. Pull out the filter and hold it toward a light source — if you cannot see light through it, replace it.
- Gas supply: Confirm your other gas appliances (water heater, stove) are working normally. If they are also out, the issue may be with your gas supply rather than the furnace itself — contact your gas company. If you smell gas at any point, leave the house first and call from outside.
- If the furnace has locked out: Set the thermostat to OFF, note the blinking error code (next section), and call a technician. Do not cycle the power or press reset buttons; lockout is the furnace protecting itself.
What Do Furnace Error Codes Mean?
Many modern furnaces have a diagnostic LED on the control board that blinks fault codes when the furnace shuts down. This is one of the most valuable pieces of information you can have before calling for service — it can help a technician identify the likely cause before even arriving, allowing them to bring the correct replacement parts.
On many models the LED is visible through a small window in the furnace cabinet. Only read it from outside; do not remove any panel to see it. To read it:
- Wait for the furnace to complete its lockout sequence (stop attempting ignition)
- Watch the LED pattern: count the number of short blinks, any pause, and any long blinks. For example: 3 short blinks, pause, 1 long blink = code 3-1
- Photograph the blinking pattern if possible, or write down the count
- Look up the code in your furnace owner's manual or on the manufacturer's website. If the code chart is only inside the cabinet, leave it for the technician and just report the blink pattern.
Reporting the error code when you contact a provider can significantly speed up diagnosis and allow the technician to bring the most likely replacement parts on the first visit — reducing the chance of a return trip and getting your heat restored faster.
Common furnace error codes and what they typically indicate:
| Common Code Description | Likely Component | Urgency |
|---|---|---|
| Pressure switch open / not closing | Pressure switch, inducer motor, or blocked condensate | Same day |
| Ignition failure / no flame | Hot surface igniter or gas valve | Same day |
| Flame sensor fault / flame lost | Dirty or failed flame sensor | Same day |
| High limit switch open | Overheating — check filter first; could be heat exchanger | Urgent |
| Roll-out switch tripped | Flame rollout — potential heat exchanger or gas issue | Emergency — do not reset |
Do I Need Emergency or Standard Furnace Service?
Gas smell or suspected gas leak (leave the building now, then call 911 or your gas utility’s emergency line from outside; see NFPA gas-leak guidance) • Roll-out switch tripped error code • CO alarm activating (get everyone outside first) • Freezing weather with infants, older adults, or pets at home • Indoor temperature already below 55°F, the minimum the American Red Cross advises keeping a home at to help prevent frozen pipes
Furnace completely non-functional • Cracked heat exchanger diagnosis • Error code indicating ignition or gas valve failure • System trying to ignite but repeatedly locking out
Intermittent heating issues • System heating but with unusual noises • Reduced efficiency or uneven heating across rooms • Furnace running but not reaching set temperature in mild weather
How Do I Stay Safe When My Furnace Fails?
While waiting for service, maintaining safe temperatures inside your home is the priority. These steps can protect occupants and your home's plumbing:
- Electric space heaters: Safe to use in occupied rooms. Keep a minimum 3-foot clearance around the heater and never leave running unattended or while sleeping. Do not use extension cords — plug directly into a wall outlet.
- Never use these for indoor heating: Gas ovens or stovetops (carbon monoxide risk), outdoor propane heaters (CO risk in enclosed spaces), charcoal grills (CO risk), or generators run indoors or in attached garages (CO risk).
- Prevent frozen pipes: The American Red Cross advises letting cold water drip from faucets served by exposed pipes when it is very cold outside, and opening kitchen and bathroom cabinet doors so warmer air can reach the plumbing.
- Consolidate to warmer rooms: Close off unneeded rooms and consolidate household members to a smaller area where a space heater can maintain safer temperatures more efficiently.
- Check on vulnerable people: Cold exposure is particularly dangerous for older adults, infants, and people with cardiovascular conditions. Watch for shivering, confusion, slurred speech, or drowsiness — signs of hypothermia that require immediate medical attention.
- Consider a warming center if temperatures are extreme: If you cannot keep the home warm and have vulnerable household members, a hotel, family member's home, or community warming center may be the safer choice while waiting for repairs.
Should I Repair or Replace My Furnace?
The repair-or-replace decision is frequently presented to homeowners during a winter emergency, when pressure to restore heat quickly can cloud the financial analysis. The points below are rules of thumb, not industry standards, and none replaces a technician's diagnosis:
Repair usually makes sense when:
- The furnace is 15 years old or younger and the repair involves a single component (igniter, flame sensor, pressure switch)
- The heat exchanger is confirmed intact
- The repair would cost well under half the price of a comparable new furnace
- The system has not needed frequent repairs recently
Get a replacement quote when:
- The heat exchanger is cracked — the repair ($600–$4,000 per our cost guide) can approach the price of a new gas furnace ($2,500–$6,500 installed), and the other components are equally aged
- The furnace is more than 15 years old, the age at which ENERGY STAR suggests considering replacement
- The repair would cost more than half the price of a comparable new furnace, or the furnace's age in years multiplied by the repair quote is more than $5,000 (the “$5,000 rule”)
- The furnace needs frequent repairs and your energy bills are going up (another ENERGY STAR replacement signal)
- You want better efficiency: ENERGY STAR says a qualified furnace is 15% more efficient than a conventional furnace
Frequently Asked Questions
This is a built-in safety lockout. Modern furnaces make a limited number of ignition attempts before entering lockout mode, to prevent unburned gas from accumulating in the combustion chamber. Once locked out, the furnace records an error code and will not try again until it is reset. The number of attempts and the lockout behavior vary by manufacturer and model. Leave the reset to a technician: set the thermostat to OFF, note the error code, and call.
Per our cost guide: igniter replacement $100–$300; flame sensor cleaning or replacement $75–$250; gas valve $200–$600; draft inducer motor $200–$700; control board $300–$900; heat exchanger repair $600–$4,000, and a new gas furnace runs $2,500–$6,500 installed. A diagnostic or service-call fee typically runs $65–$150 (some providers credit it toward the repair if you book it with them; ask when you call), and night, weekend and holiday calls add $100–$300 on top of the repair cost.
You cannot rely on smell. The main danger from a cracked heat exchanger is carbon monoxide, which is colorless and odorless. Per the CDC, the most common symptoms of CO poisoning are headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion. If you suspect CO poisoning, CPSC advises getting outside to fresh air immediately and calling the fire department from a neighbor's home. A working CO alarm is your best protection.
We recommend leaving resets to a technician. If the furnace has locked out, set the thermostat to OFF, note the error code, and call. Do not reset breakers, flip the furnace's power switch, or press reset buttons yourself: lockout is the furnace protecting itself from a fault, and repeatedly restarting a failing furnace can hide a developing safety hazard.
DOE's federal purchasing guidance assumes an average service life of 21 years for a residential furnace. That is a life-cycle-cost modelling assumption, not a promise of how long any one unit will last. ENERGY STAR suggests considering replacement once a furnace is more than 15 years old, and recommends a contractor check-up of the heating system every fall.
Two rules of thumb (not industry standards) can help: get a replacement quote if the repair would cost more than half the price of a comparable new furnace, or if the furnace's age in years multiplied by the repair quote is more than $5,000. ENERGY STAR suggests considering replacement when a furnace is more than 15 years old, or when it needs frequent repairs while your energy bills are going up. A cracked heat exchanger is a common reason to weigh replacement, because the repair ($600–$4,000 per /costs) can approach the price of a new gas furnace ($2,500–$6,500 installed). For a full decision framework including the $5,000 rule, SEER2 breakeven analysis, and a decision matrix, see our HVAC repair vs. replace guide.
If you’ve checked the thermostat and filter and the furnace is still locked out, the next step is a technician. Have the error code, the furnace’s age, and when the problem started ready when you call.
Call Now — (844) 582-1795Disclosure: We are a referral service and may receive compensation for qualified calls. Calls may be routed to an independent provider network and may be recorded. Pricing and availability vary by provider and location.
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