If you smell gas at any concentration: leave the house, call the gas utility from outside. Not your HVAC guy first. The utility. Don't flip light switches, don't open the panel, don't try to "find the leak." Get out. Then call.
How a modern furnace actually starts (the sequence of operations)
This is the chain. If it breaks, it breaks at one of these steps, in this order:
- Thermostat closes R-W circuit — sends 24V call for heat to the control board.
- Inducer motor energizes — draws combustion air, purges the heat exchanger. Fails here? Bad inducer motor, bad control board, broken wire.
- Pressure switch closes — inducer's vacuum pulls the diaphragm shut, proving draft. Fails here? Blocked vent, blocked condensate trap on a 90%+ unit, cracked switch hose, dead switch, weak inducer.
- Hot Surface Igniter glows — board sends 120V to the HSI, it goes cherry orange for 15-45 seconds. Fails here? Cracked igniter (most common wear part), open igniter circuit, bad board.
- Gas valve opens — board energizes the valve solenoid, gas flows to burners. Fails here? Closed gas cock, low manifold pressure, dead valve, no 24V at valve.
- Flame establishes and flame sensor proves it — the rod sits in the flame, micro-amp current flows (typically 2-6 μA on a clean rod). Fails here? Dirty flame sensor (the big one), cracked porcelain, bad ground, weak ignition.
- Blower delay (30-60 sec), then circulation blower starts. Fails here? Bad limit switch (overheat), board issue, blower capacitor.
If the blower runs but no heat — you broke the chain at step 3, 4, 5, or 6, and the board ran the post-purge blower anyway to cool the heat exchanger.
Top 7 causes ranked by what I actually see
| # | Cause | Why it tops the list | Symptom |
|---|---|---|---|
| 1 | Dirty flame sensor | 60-80% of "no heat" winter calls. Carbon insulates the rod, micro-amp signal drops below threshold, board shuts gas in 5-10 sec. | Lights, dies. Lights, dies. Three tries → lockout. |
| 2 | Cracked / weak HSI igniter | Silicon nitride rods are wear parts — 5-8 year life, shorter if the unit short-cycles. | No glow at all, or weak orange. No ignition sequence at burners. |
| 3 | Clogged condensate trap (90%+ units) | Acidic condensate grows slime in the P-trap. Water backs up, pressure switch can't close. | Inducer runs forever, never advances to igniter. 3-flash Carrier, 2-flash Goodman. |
| 4 | Bad pressure switch / blocked vent | Switch fails open with age, or vent is iced over / bird-nested / blocked. | Same as #3 — inducer runs, no progression. |
| 5 | Gas valve issue | Coil dies, manifold pressure drifts low, or valve sticks. Less common than people think. | HSI glows, no flame at all, faint click of solenoid. |
| 6 | Control board | Cracked solder joints, fried relays. Diagnosis-by-elimination. | Erratic behavior, dead board lights, intermittent everything. |
| 7 | Thermocouple (standing-pilot units, pre-1990s) | Only applies if you actually have a standing pilot — most modern furnaces don't. | Pilot won't stay lit when you release the gas knob. |
The 30-minute homeowner self-diagnosis
Things you can do without dying:
- Thermostat — set to HEAT, 5°F above room temp. Replace batteries.
- Breaker — check the panel. Furnace is usually a dedicated 15A.
- Furnace switch — looks like a light switch on the side of the unit. Make sure it's ON.
- Filter — pull it. If you can't see light through it, that's your problem. Run with no filter for one cycle just to test.
- Gas cock — yellow handle parallel to the pipe = open. Make sure your stove still lights (proves house gas is on).
- Condensate float switch (90%+ units) — a tripped float kills the call for heat. Find it on the condensate pump or trap.
- Flame sensor cleaning — this is the fix that resolves the most calls. Power OFF at switch. Pull burner door. Flame sensor is a single rod with one wire, sitting in the burner flame path. One screw. Pull it. Polish gently with fine emery cloth or a folded dollar bill until shiny. Reinstall. Power on. Cycle the thermostat.
- Watch the LED through the sight glass — count the flashes. Write it down. See the table below.
If steps 1-8 don't fix it: stop. You've done the homeowner work. See "when to call a pro" below.
Error-code table — verified from manufacturer documentation
| Brand | Flash code | What it actually means |
|---|---|---|
| Carrier / Bryant | 3 flashes (Code 33 on 2-digit) | Pressure switch didn't close / opened during run. Check condensate trap, vent, hose, inducer. |
| Carrier / Bryant | 1-3 (limit) | Limit circuit fault — typically overheat from clogged filter or blocked supply registers. |
| Carrier / Bryant | 3-4 (no flame) | Igniter or gas valve didn't get flame on retry. |
| Lennox SureLight | Slow flash + diag code 1 | Pressure switch open / blocked vent / condensate. |
| Lennox SureLight | Alternating slow flash | Watchguard — repeated ignition failures, board in lockout. |
| Trane | 2 flashes | System lockout — failed to ignite after 3 attempts. Igniter, gas, or flame sense. |
| Goodman / Amana / Daikin | 1 flash | Ignition failure or flame failure. |
| Goodman / Amana / Daikin | 3 flashes | Pressure switch stuck open. |
| Goodman / Amana / Daikin | 4 flashes | Open high-limit switch (overheat). |
| Rheem / Ruud | 1 blink (2-sec pause) | Soft lockout — 1 hour timeout, usually from failed ignition. |
| Rheem / Ruud | 3 flashes | Pressure switch fault. |
Source: manufacturer service manuals — Carrier Status Code chart, Lennox SureLight tech literature 0222-L7, Trane diagnostic guide, Goodman fault code chart, Rheem PTS 92-103343-01.
When to call a pro — no negotiation
- You smell gas. Anywhere. Any concentration. Leave the house, call the gas utility from outside. Not the HVAC guy. The utility.
- The unit locks out repeatedly after you cleaned the flame sensor.
- No glow at the HSI igniter on a call for heat — replacement needs the right part number and a continuity check.
- Error code doesn't clear after a power-cycle reset.
- Yellow flame, soot around the cabinet, or the CO alarm has ever gone off. Combustion problem. Pro with a combustion analyzer.
- Anything involving the gas valve, manifold pressure, the heat exchanger, or the vent pipe. Licensed-tech jobs. A wrong manifold pressure setting or a cracked heat exchanger kills people every winter.
FAQ
Why does my furnace click but not turn on?
Usually the HSI igniter trying to glow but failing, or the gas valve solenoid clicking without gas flow. Watch for the orange glow inside — no glow means igniter or board.
Why does my furnace blower run but no heat comes out?
The board failed to prove flame, killed the gas, but still runs the post-purge blower to cool the heat exchanger. You're hearing the safety system work correctly.
How long do furnace igniters last?
5-8 years on silicon nitride HSIs. Less if your unit short-cycles. They crack from thermal stress, not voltage.
Can I clean my own flame sensor?
Yes — power off, pull the rod, fine emery cloth or dollar bill, no steel wool. ~10-min job, fixes most "ignites then dies" calls.
Why does my furnace start, run a minute, then shut off?
Classic dirty flame sensor. The flame lights, the rod can't read it through the carbon layer, board kills gas as safety. Three retries then lockout.
Field-pattern callout — what about half of homeowners learn the hard way
"Igniter glows, burners light for about 3 to 5 seconds, then gas shuts off. Did it three times then I got a flashing light. Cleaned the flame sensor with emery cloth — back to running. Then 3 weeks later same thing. Cleaned again, fine."
Al's read: Three-week return means it's not just dirt — it's the burner face going crusty or the rod itself starting to fail. Once you've cleaned the same sensor twice in a season, you replace it. $15 part. Don't keep buffing — abrasive cleaning eats the rod and shortens its life every time.
Related buying guides
- Best universal furnace igniters 2026 — silicon nitride vs carbide, brand compatibility
- Best smart thermostats 2026 — when the thermostat is the problem
- Furnace banging when starting — separate symptom, related diagnostics
Editorial standards: Cited authorities include manufacturer service literature for Carrier, Bryant, Lennox SureLight (0222-L7), Trane, Goodman/Amana/Daikin, and Rheem/Ruud (PTS 92-103343-01). NFPA 54 (National Fuel Gas Code) governs gas-fired install. Reviewed by Al, Building Doctor — IUOE Local 39 Stationary Engineer, 30 years commercial gas heating.