Furnace Won't Start: 10 Causes, Blink Codes by Brand, and What to Fix Yourself
If your furnace is not starting, you have either a simple fix that takes five minutes or a safety issue that requires a technician today. The two most common causes — a tripped breaker and a clogged filter — cost nothing and take minutes. The most dangerous — a gas odor or a cracked heat exchanger — require you to leave the home and call immediately.
Before You Start: Gas Odor and CO Rules
If you smell gas inside the home: do not touch any switches. Leave immediately. Call your gas utility or 911 from outside.
If your CO detector is alarming: evacuate all occupants immediately. Call 911 from outside.
Start With the Free Checks
Run these four checks before spending anything or calling anyone. Two of the three most common causes of a furnace not starting are in this list.
Pull the filter and replace it if clogged ($10–$30). Confirm the thermostat is set to Heat with the setpoint above the current room temperature, and replace the batteries. Check the circuit breaker — reset it once if tripped (do not reset a repeatedly tripping breaker). Find the furnace power switch (looks like a standard light switch on the unit or wall) and confirm it is ON.
The Most Replaced Part: Hot Surface Igniter
Most modern furnaces use a hot surface igniter — a fragile silicon carbide or silicon nitride element that glows orange-hot to ignite the gas. Silicon carbide igniters typically last 3–7 years before cracking. When they fail, the furnace cycles through the startup sequence but gas never ignites. You may hear the inducer fan run, then silence where ignition should happen.
The igniter is extremely fragile — touching it with bare hands leaves oils that cause cracking. If you are not experienced with furnace components, have a technician replace it ($150–$400).
The Safety Risk You Cannot Ignore: Carbon Monoxide
Accidental CO poisoning deaths in the U.S. reached 624 in 2022 — up 85.7% from 2012, according to CDC data. The CPSC estimates over 100,000 emergency room visits annually from CO exposure. 84% of consumer-product CO deaths happen September through April — peak furnace season.
A cracked heat exchanger separates combustion gases (including CO) from the air circulating through your home. When it cracks, those gases can mix with conditioned air. Never inspect or continue operating a furnace with a suspected cracked heat exchanger.
How to Read Your Furnace's Blink Codes
When the furnace shuts off from an error, the control board LED flashes a code. Count the flashes before the pause — that number tells you the fault category. The code chart is always printed on a sticker inside the furnace access panel.
The Codes You Must Not Reset: Code 5 and Code 9
| Code | Meaning | Action |
|---|---|---|
| Code 5 (all brands) | Flame sensed with no heat call — gas valve may be leaking | Shut off gas; call technician same day |
| Code 9 / Rollout (Goodman, Rheem, York) | Flame exiting combustion chamber — fire hazard | Do NOT reset. Call technician immediately. |
| Code 6 (York/Coleman) | Rollout switch open | Do NOT reset. Call technician immediately. |
Blink Code Reference by Brand
| Blinks | Carrier/Bryant | Trane/Am. Std. | Goodman/Amana | Rheem/Ruud | York/Coleman |
|---|---|---|---|---|---|
| 2 | Ignition lockout | System lockout | Inducer not running | Pressure switch open | Pressure switch closed |
| 3 | Pressure switch open | Vent pressure error | Pressure switch open | Pressure switch closed | Pressure switch won't close |
| 4 | Flame loss fault | High temp limit open | High limit open | High limit open | Limit switch open |
| 5 | Limit switch open | Flame with no call ⚠ | Flame with no call ⚠ | Flame with no call ⚠ | — |
| 8 | Module fault | Low flame signal | Ignitor failure | Ignitor failure | — |
| 9 | — | — | Rollout switch ⚠ | Rollout switch ⚠ | — |
⚠ = Do not reset. These indicate active safety hazards requiring a licensed technician.
2026 Repair Cost Reference
| Component | Parts Cost | Installed Total |
|---|---|---|
| Filter | $10–$30 | DIY |
| Flame sensor cleaning | $0 | $75–$170 |
| Flame sensor replacement | $10–$30 | $85–$250 |
| Hot surface igniter | $15–$100 | $150–$400 |
| Pressure switch | $50–$150 | $150–$400 |
| Control board | $50–$400 | $250–$650 |
| Blower motor (single-speed) | $100–$300 | $300–$600 |
| Blower motor (variable-speed ECM) | $300–$500 | $500–$800+ |
| Heat exchanger | $350–$1,500 | $1,000–$3,500 |
Source: HVAC Pricing Guide (March 2026), HVAC Calculator Hub (July 2026), HomeGuide (2026).
When to Call a Technician Today
- The furnace attempts ignition but shuts off every time — and one manual reset did not fix it.
- You have Code 5 on any brand (flame sensed without a heat call — gas valve issue).
- You have Code 9 or Rollout switch on any brand — do not reset, call now.
- The breaker trips again after a single reset.
- You have CO detector readings — even low, non-alarming readings.
- The furnace runs briefly, produces some heat, then shuts off repeatedly.
Read the blink code before calling — tell the technician the code number when you schedule. It tells them what to bring on the truck and cuts diagnostic time significantly.
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