Check This First — It Takes 30 Seconds
Before diagnosing anything, check your thermostat fan setting. If the fan is set to ON instead of AUTO, the blower runs continuously — including between heating cycles when the heat exchanger is cold. You get cold air from the vents even when the furnace is otherwise operating correctly. Switch it to AUTO. If warm air returns within 5–10 minutes, this was your only problem.
If the fan is set to AUTO and you still get cold air, the furnace is either not igniting or is shutting down early. Everything below covers those scenarios.
10 Causes — Diagnose in This Order
1. Thermostat Set to Fan ON (Not a Furnace Problem)
As above. Switch to AUTO and wait 10 minutes. If this fixes it, no further diagnosis needed.
2. Dirty Air Filter — Limit Switch Trips
A severely clogged filter restricts airflow, causing the heat exchanger to overheat. The high-limit switch trips, cutting the burner to prevent damage — but the blower continues running on cool air. The furnace may restart after cooling down, then trip again. This is the most common reason for a furnace running but not heating. Replace the filter (1" filter: replace every 30–90 days; 4" media filter: every 6–12 months). If a new filter fixes the problem, clean the air handler interior — a dirty system loaded the filter faster than it should.
3. Ignitor Failure — No Flame at All
Most modern furnaces (manufactured since the mid-1990s) use a silicon nitride or silicon carbide hot surface ignitor instead of a standing pilot light. Silicon carbide ignitors glow at approximately 2,000–2,500°F; silicon nitride (now more common) runs cooler at around 1,800–2,100°F — still more than sufficient to ignite natural gas, but more durable due to lower thermal stress. They are fragile and typically fail after 3–7 years. If you see the draft inducer run, then a 30-second pause, then nothing — the ignitor may have failed.
Common OEM ignitor part numbers (prices as of mid-2026; verify current pricing before ordering):
- Carrier: LH33ZG001 (~$45–$60 OEM from Carrier dealer; less from aftermarket suppliers)
- Trane: IGN00023 (verify your model — Trane has multiple ignitor part numbers)
- Lennox: 41K56 (Lennox-authorized dealer only for warranty compliance)
- Universal replacement: Supco IG403K (roughly $19–$30 on Amazon at time of writing) — fits many residential furnaces but verify compatibility with your unit's specifications before ordering
Ignitor replacement is a realistic DIY repair if you are comfortable with the electrical access and can match the part specification. Turn off the furnace breaker, remove the ignitor mounting screw (one screw), swap the part, reinstall. Handle the new ignitor by the wiring harness only — skin oils on the ceramic element can cause premature failure.
4. Dirty Flame Sensor — Burner Lights Then Shuts Off in 3–5 Seconds
The flame sensor is a metal rod that confirms the burner has lit by passing a microamp current through the flame. A coating of oxidation (white residue) insulates the rod and prevents it from sensing the flame. The control board shuts the burner down as a safety measure, then attempts restart 1–3 times before lockout. You may hear the burner ignite briefly then go quiet.
Cleaning a flame sensor is a legitimate DIY maintenance task. Turn off the furnace, locate the sensor (single metal rod with a ceramic base, positioned directly in the flame path), remove the one mounting screw, and lightly abrade the metal rod with fine steel wool or 0000 emery cloth for 5–10 seconds. Reinstall. This is the $0 fix that resolves roughly 20–30% of "furnace won't stay lit" calls.
5. Gas Valve Not Opening
If you can hear the ignitor glow but see no flame, the gas valve may have failed or the gas supply may be interrupted. Check other gas appliances (water heater, stove). If those work, the issue is specific to the furnace's gas valve or a closed manual gas shutoff valve. Gas valve replacement: $300–$600 installed. Never attempt to repair a gas valve — replacement only.
6. Pressure Switch Tripped — Draft Inducer Problem
Modern condensing furnaces use a draft inducer motor to pull combustion gases through the heat exchanger before venting. A pressure switch confirms sufficient negative pressure (typically -0.5 to -1.5 inches water column) before allowing the burner to fire. If the inducer is failing, the condensate drain is blocked, or the pressure switch itself has failed, the furnace will not initiate combustion. Symptom: you hear the inducer run, but nothing happens after that. Inducer replacement: $350–$850 installed. Pressure switch replacement: $75–$200 installed.
7. Control Board Failure
The control board is the furnace's central processing unit — it sequences every component from draft inducer to gas valve to blower motor. Boards fail due to power surges, capacitor aging, and manufacturing defects. A board failure often shows up as erratic operation, components that respond inconsistently, or a blank diagnostic LED. Replacement: $250–$600 for the board, $350–$900 installed total.
8. Rollout Switch Tripped
A manual reset rollout switch is a safety device that trips when flames "roll out" of the combustion chamber — an indication that the heat exchanger may be cracked, the gas pressure is too high, or the secondary heat exchanger is blocked. Unlike the limit switch, the rollout switch requires manual reset (a button on the body of the switch). If it trips repeatedly, the underlying cause must be fixed before continuing to operate the furnace. A rollout switch that keeps tripping should be treated as a potential cracked heat exchanger indicator.
9. Old Standing Pilot Light (Pre-1990s Systems)
If your furnace predates approximately 1992, it may use a standing pilot light instead of an electronic ignitor. A pilot that goes out causes no heating — but the blower may still run if the room thermostat calls for it. Relighting the pilot: follow the instruction label on the furnace cabinet door. If the pilot won't stay lit after 3 attempts, the thermocouple has likely failed ($20–$60 to replace).
10. Cracked Heat Exchanger — Emergency
This is the one that matters. The heat exchanger is the metal assembly separating combustion gases (which contain carbon monoxide) from the air distributed through your home. A crack allows combustion byproducts — including CO — to enter the living space. The CDC reports approximately 420 deaths and 100,000 emergency room visits from unintentional CO poisoning annually in the United States; a significant portion involve heating equipment.
Symptoms of a cracked heat exchanger:
- Headaches, nausea, or dizziness that improve when you leave the house
- CO detector alarm
- Visible soot or scorching on furnace components beyond the combustion chamber
- Strong odor at first startup (different from normal dust-burn smell at beginning of season)
- Rollout switch that trips repeatedly
- Flames that flutter or move when the blower engages
What to do: Shut off the furnace. Open windows. Leave the house. Call your gas utility's emergency line and a licensed HVAC contractor for inspection. Do not restart the furnace until a technician has visually confirmed no crack.
Repair vs. replace: Heat exchanger replacement on an aging furnace costs $1,000–$3,000 for the part alone, plus labor. On a furnace over 15 years old, this almost always exceeds 50% of replacement cost. The industry standard recommendation is to replace the furnace when a heat exchanger crack is found on an older system, not repair it.
Read Your Furnace's Blink Codes
Every modern furnace with a circuit board has a diagnostic LED that blinks a fault code when it enters lockout or safe mode. Look through the small inspection window on the lower access panel — you will see a red or amber LED. Count the blinks (or flash pattern). The code is also printed on a legend inside the access panel door.
These are the most commonly reported codes for each brand. Blink codes vary by model year and control board — always cross-reference with the legend printed on your unit's access panel door. Use this table as a starting point, not a definitive diagnosis.
| Brand | Blink / Code | Meaning | Likely Cause |
|---|---|---|---|
| Carrier | 2 blinks | Ignition failure / lockout | Ignitor, gas valve, gas supply, flame sensor |
| 3 blinks | Draft pressure fault | Draft inducer, flue blocked, pressure switch | |
| 4 blinks | Open high limit device | Dirty filter, blocked vents, low airflow | |
| 5 blinks | Flame without call (rollout) | Cracked heat exchanger, rollout switch tripped | |
| Trane / American Standard | 2 blinks | System lockout | Multiple failed ignition attempts |
| 3 blinks | Pressure switch open | Draft inducer, condensate blockage, flue | |
| 4 blinks | Open high-temperature limit | Dirty filter, blocked return, low airflow | |
| Goodman / Amana | E0 | Lockout after ignition retries | Ignitor, gas valve, flame sensor |
| E1 | Ignition failure | Ignitor or gas supply | |
| A0 | Dirty flame sensor | Flame sensor oxidation — clean it first | |
| E6 | Inducer motor fault | Draft inducer, pressure switch | |
| Rheem / Ruud | 1 flash | Ignition lockout | Ignitor, gas supply, flame sensor |
| 2 flashes | Pressure switch stuck open | Draft inducer, blocked condensate | |
| 4 flashes | High limit switch open | Dirty filter, blocked airflow | |
| Lennox | Code 10 | Soft lockout | Multiple failed starts; check ignitor and gas |
| Code 34 | Ignition failure | Ignitor, gas valve, spark gap | |
| Code 45 | Low flame signal | Dirty flame sensor — clean first | |
| York / Coleman | 7 flashes | Flame sensor fault | Dirty flame sensor |
| 3 flashes | Pressure switch open | Draft inducer, flue restriction | |
| Bryant 926T | Code 13 | High limit lockout | Dirty filter, poor airflow, overheating |
| Code 24 | Secondary voltage fault | Low-voltage wiring issue | |
| Code 45 | Control board lockout | Multiple safety trips; board may be failing |
When to Call Immediately vs. Safe to Wait
| Situation | Action |
|---|---|
| CO detector alarm | Evacuate immediately. Call 911 from outside. Do not re-enter until cleared by fire department. |
| Smell of gas (rotten egg odor) | Do not touch any switches. Leave immediately. Call gas utility from outside. |
| Rollout switch tripping repeatedly | Shut off furnace. Call HVAC technician same day for cracked heat exchanger inspection. |
| Thermostat fan ON / AUTO setting | Fix it yourself in 30 seconds. |
| Dirty filter causing limit trip | Replace filter now. System may recover without a service call. |
| Dirty flame sensor (burner lights then dies) | Clean it yourself if comfortable, or schedule next-day service. |
| Ignitor failure | Replace yourself or schedule non-emergency service. (Safe, no CO risk.) |
| Pressure switch / inducer fault | Schedule service within 1–2 days. No safety risk if furnace is off. |
| Control board failure | Schedule service. Not a safety emergency. |
Carbon monoxide is odorless and colorless. At 70 ppm, symptoms begin in 2–3 hours for healthy adults (OSHA PEL is 50 ppm over 8 hours). At 400 ppm, headache, dizziness, and nausea develop within 1–2 hours. At 1,600 ppm, headache and dizziness within 20 minutes, and death within 2 hours (CDC). A $25 CO detector is non-negotiable if you have any gas appliance.
Repair Cost Summary for Furnace Issues
| Repair | DIY Cost | Installed (Professional) |
|---|---|---|
| Air filter replacement | $8–$40 | N/A (do this yourself) |
| Flame sensor cleaning | $0 | $75–$150 (included in service call) |
| Hot surface ignitor | $20–$55 (part) | $150–$350 installed |
| Thermocouple (standing pilot) | $10–$30 (part) | $75–$175 installed |
| Pressure switch | — | $75–$200 installed |
| Draft inducer motor | — | $350–$850 installed |
| Gas valve | — | $300–$600 installed |
| Control board | — | $350–$900 installed |
| High limit switch | — | $100–$250 installed |
| Heat exchanger replacement | — | $1,000–$3,000+ (often warrants full replacement) |
| Full furnace replacement (80% AFUE) | — | $2,500–$5,500 installed |
| Full furnace replacement (96% AFUE) | — | $3,500–$7,500 installed |
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