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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 is usually caused by a failed igniter ($100–$300 to replace), dirty flame sensor ($65–$150 to clean), or faulty pressure switch ($75–$250). A cracked heat exchanger is the most serious failure at $600–$2,500, and often means full furnace replacement ($2,500–$6,000). If you smell gas, leave the house immediately and call your gas company.
It's a January night, the temperature outside has dropped to 18°F, and you hear your thermostat click 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.
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 to approximately 2,000°F over 30 to 60 seconds, 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 typically retries 2 to 3 times 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 the most frequently replaced furnace component. It is a fragile ceramic element that becomes extremely brittle after years of thermal cycling (heating to 2,000°F and cooling hundreds of times per season). 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 you hear: The inducer motor runs (a whooshing sound), there may be a brief click, but no combustion sound follows. The furnace attempts this 2 to 3 times, then goes silent with a blinking error light.
Igniter failure is particularly common at the start of heating season after months of inactivity, or after a power surge. It is among the less expensive furnace repairs and is typically completed within 1 to 2 hours.
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 2 to 5 seconds, then shuts off — because the dirty sensor cannot confirm that flame is present.
What you hear: The furnace ignites briefly — you may feel a small pulse of warm air — then shuts off. It attempts this cycle 3 times before lockout.
In many cases, a professional can clean the flame sensor rod with fine steel wool ($65–$150) rather than replacing it. If cleaning does not restore reliable operation, replacement is $150 to $250. This is one of the most cost-effective furnace repairs.
3. Faulty Pressure Switch — $75–$250
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 you hear: 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.
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. Attempting this yourself may also void your homeowner's insurance and violates most local building codes.
5. Cracked Heat Exchanger — $600–$2,500 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.
When the heat exchanger develops a crack (caused by metal fatigue from years of thermal expansion and contraction, or overheating from a clogged filter), modern furnaces detect the resulting pressure differential and refuse to ignite as a protective measure. This is the furnace working correctly — protecting you from a CO hazard.
Signs beyond ignition failure: Yellow or flickering pilot 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. The repair cost ($600–$2,500) often approaches or exceeds a significant portion of a new furnace cost — making full system replacement the financially rational choice on furnaces older than 15 years. 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 — $300–$800
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 typical ranges for the most common furnace repairs. Actual costs vary by region, provider, and whether emergency service is needed. A diagnostic service fee of $65–$150 is typically charged at the start of a call and applied toward the repair if work proceeds.
| Component | Repair / Service | Typical Cost | Time |
|---|---|---|---|
| Hot surface igniter | Replacement | $100–$300 | 1–2 hrs |
| Flame sensor | Cleaning | $65–$150 | 1 hr |
| Flame sensor | Replacement | $150–$250 | 1 hr |
| Pressure switch | Replacement | $75–$250 | 1–2 hrs |
| Gas valve | Replacement | $200–$600 | 2–3 hrs |
| Inducer motor | Replacement | $300–$800 | 2–4 hrs |
| Control board | Replacement | $300–$900 | 2–3 hrs |
| Heat exchanger | Repair | $600–$2,500 | 4–8 hrs |
| Full furnace replacement | New unit + installation | $2,500–$6,000 | 1 day |
| Emergency / after-hours premium | Added to any call | +$75–$200 | — |
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.
- Circuit breaker: Locate the furnace's dedicated breaker in your electrical panel. If it has tripped to the center or OFF position, reset it once: flip it fully to OFF, then back to ON. Wait 5 minutes before testing. If the breaker trips again, do not reset it again — a repeatedly tripping breaker indicates an electrical fault requiring professional attention.
- Furnace power switch: A standard-looking light switch is typically mounted on the wall near the furnace or on the unit itself. It is easy to accidentally turn off, especially in utility areas with multiple switches. Confirm it is in the ON position.
- 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 before attempting to restart.
- 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.
- Attempt one reset: Turn the furnace power switch off, wait 30 seconds, and turn it back on. If the furnace ignites successfully and runs through a full cycle, monitor it carefully over the next few hours. If it locks out again, do not keep resetting it.
What Do Furnace Error Codes Mean?
Most furnaces manufactured in the past 20 years 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.
The LED is typically visible through a small window or vent on the furnace cabinet. 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 (usually taped inside the cabinet door) or search your furnace model number plus the error code online
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 building, call 911 first) • Roll-out switch tripped error code • CO alarm activating • Outdoor temperature below 20°F with infants, elderly, or pets • Home temperature already below 55°F
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: If indoor temperatures fall toward 50°F, let cold water faucets drip slightly on both floors. Open cabinet doors under sinks on exterior walls to allow warmer air circulation around pipes. Know where your main water shutoff is located.
- 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 maintain indoor temperatures above 55°F 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. Here is a clear framework:
Repair is almost always the right choice when:
- The furnace is under 15 years old and the repair involves a single component (igniter, flame sensor, pressure switch)
- The heat exchanger is confirmed intact
- The repair cost is under 30–40% of a new furnace's installed cost
- The system has not required multiple significant repairs recently
Replacement deserves serious consideration when:
- The heat exchanger is cracked — repair cost often approaches a new system cost, and other components are equally aged
- The furnace is 20 or more years old
- The repair cost exceeds 50% of the installed cost of a new furnace
- Multiple significant components have failed in recent seasons
- The current furnace is 60–70% AFUE; modern units at 80–98% AFUE can reduce heating costs by 20–35% annually — savings that meaningfully offset replacement cost over time
Frequently Asked Questions
This is a built-in safety lockout. Modern furnaces attempt ignition 2 to 3 times before entering lockout mode to prevent unburned gas from accumulating in the combustion chamber — which would create an explosion risk. Once locked out, the furnace records an error code and will not attempt ignition again until manually reset. The number of attempts and lockout behavior varies by manufacturer and model.
Common repairs: igniter replacement runs $100–$300; flame sensor cleaning $65–$150, replacement $150–$250; pressure switch $75–$250; gas valve $200–$600; control board $300–$900; cracked heat exchanger $600–$2,500 or prompts full replacement at $2,500–$6,000. Emergency service adds $75–$200. A diagnostic fee of $65–$150 is typically applied toward the repair if work is performed.
A cracked heat exchanger is usually undetectable by smell — carbon monoxide is colorless and odorless. A faint formaldehyde-like chemical odor may be present in some cases. More reliable indicators are physical symptoms: frequent headaches, dizziness, or nausea that clear when leaving the house. If you suspect CO exposure, open windows, leave the building, and call emergency services and a professional immediately. A working CO detector is your best protection.
You can safely attempt one reset after lockout. Turn the furnace power switch off, wait 30 seconds, and turn it back on. If the furnace ignites and runs normally, monitor it through the next few cycles. If it locks out again, do not continue resetting it — repeated lockouts indicate an underlying issue that requires professional diagnosis. Continually resetting a failing furnace can mask a developing safety hazard.
A well-maintained gas furnace lasts 15 to 30 years, with most units averaging around 20 years. Annual professional tune-ups, regular filter changes, and prompt attention to minor repairs extend lifespan significantly. After 20 years, even maintained furnaces see sharply rising repair frequency and declining heat exchanger integrity, making replacement increasingly cost-effective.
If the repair cost exceeds 50% of a new furnace's installed cost, replacement is generally the better decision. Situations that almost always favor replacement: a cracked heat exchanger, a furnace 20 or more years old, or multiple significant component failures in recent seasons. A new furnace also typically carries a major efficiency improvement — older units may be 60–70% AFUE while current models reach 80–98% AFUE, meaning substantially lower monthly heating bills. For a full decision framework including the $5,000 rule, SEER2 breakeven analysis, and a decision matrix, see our HVAC repair vs. replace guide.
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