Before calling a furnace tech, work through this diagnostic order. Most furnace no-heat calls are resolved by the homeowner before the tech arrives — and knowing what you checked tells the tech exactly where to start when you do call.
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Diagnostic order: start here, work down
Step 1: Thermostat check (2 minutes, $0)
The most common cause of “furnace not heating” is thermostat settings or a dead thermostat battery.
- Is the system set to HEAT? Not COOL, not FAN ONLY, not OFF. This sounds obvious, but it accounts for a meaningful percentage of no-heat service calls.
- Is the setpoint above the current temperature? Set the thermostat 5°F above the current room temperature to force a heat call.
- Replace the batteries. Low or dead batteries cause thermostats to behave erratically or not send signals to the furnace. Swap them even if the display looks normal — many thermostats display “low battery” for weeks before failing.
- Check for “hold” or “away” modes on a programmable or smart thermostat. A temperature hold set to 60°F won’t trigger heating until the temperature drops below 60°F.
If the thermostat display is blank (no display at all): replace batteries first. If still blank, check the furnace power (Step 2) — the thermostat may be powered from the furnace’s 24V transformer.
If you’re seeing unfamiliar symbols on your thermostat display, see Nest thermostat symbols explained or your thermostat’s model guide — a flame icon, snowflake, or alert triangle may indicate the system mode or a specific fault.
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Step 2: Furnace power (3 minutes, $0)
The furnace needs 120V AC power and a working control circuit.
Check the breaker. Go to your electrical panel and find the breaker labeled “furnace” or “air handler.” It should be fully in the ON position — a tripped breaker sits in the middle between ON and OFF. Reset it: push fully to OFF, then push back to ON.
Check the emergency shutoff switch. Many installations have a red or standard light switch near the furnace or at the top of the basement stairs. It controls 120V power to the furnace and is easy to accidentally switch off. Make sure it’s on.
Check the door interlock. Most furnaces have a safety switch in the lower door panel — if the door isn’t fully closed, the switch cuts power to the furnace. Close the panel firmly.
Check the control board fuse. Open the lower door panel and look at the control board for a small fuse (usually labeled “3A” or “FUSE”). A blown fuse disables the furnace’s control circuit entirely. Replace it only with the same amperage. If the fuse blows again, there’s a wiring short — call a tech.
Check the LED on the control board. If there’s a fault code blinking, note it. See your brand’s flash-code guide:
- Goodman flash codes
- Carrier fault codes
- Trane fault codes
- Rheem/Ruud flash codes
- Lennox fault codes
- York/Coleman flash codes
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Step 3: Gas supply (2 minutes, $0)
Check the manual gas shutoff valve on the gas supply line leading to the furnace. The valve handle should be parallel to the gas line (open). If it’s perpendicular to the line, it’s closed — turn it parallel.
Check other gas appliances in the house. Turn on a gas burner on your stove. If it lights normally, gas supply to the house is fine. If the stove also has no gas, call your gas utility — you may have a supply outage.
Check for a gas smell. If you smell gas: do not touch any light switches or electrical devices. Leave the building, leave the door open, and call 911 or your gas utility from outside.
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Step 4: Observe a startup sequence
If power and gas are confirmed, watch the furnace attempt to start with the thermostat calling for heat. The startup sequence takes about 2–4 minutes:
- Inducer motor starts (you hear a draft motor — a fan — spin up)
- Igniter energizes (15–30 seconds for silicon carbide; 5–15 seconds for silicon nitride)
- Gas valve opens and burners light (you hear a small “whump” and see flame through the sight glass)
- Flame sensor confirms (5–10 seconds after ignition)
- Blower motor starts (typically 30–90 seconds after ignition, to allow the heat exchanger to warm up)
- Warm air blows from vents
What to note if it fails:
- Step 1 fails (no inducer): the draft motor has failed, or a pressure switch is stuck — check flash code
- Step 2 fails (no glow): the igniter has failed — see Furnace Hot-Surface Igniter: Symptoms and Replacement
- Step 3 fails (igniter glows, no flame): gas not reaching burners — gas valve issue or gas pressure
- Steps 3–4 fail (flame then shuts off in 5–10 seconds): dirty flame sensor — see Furnace Flame Sensor: Cleaning
- Step 5 fails (burners running but blower won’t start): blower capacitor or blower motor issue
- All 5 steps complete but house doesn’t heat: check airflow (Step 5 below)
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Step 5: Airflow and heat delivery
If the furnace is igniting and the blower is running but the house stays cold:
Replace the filter. A severely clogged filter limits the air the blower can move, cutting heat delivery throughout the house. Dirty enough, it will also trip the high-limit switch and shut the burners off while the blower keeps running — producing cold air from running vents.
Check for blocked vents. Walk through the house. Every supply register should be open. Closed registers in unused rooms feel like a good idea but reduce system airflow, cause the furnace to short-cycle, and in extreme cases trip the high-limit.
Check the return air path. The return air grille (usually a large grille in a hallway or central area) pulls air back to the furnace. If it’s blocked, the blower starves for air.
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When the furnace goes into lockout
After 3 failed ignition attempts, most furnace control boards lock out and display a fault code. The furnace won’t try to start again automatically for 1–3 hours.
To reset immediately: find the furnace power switch (wall switch near the furnace or toggle on the unit) and turn it off for 30 seconds, then restore. This clears the lockout.
Before resetting: diagnose why it failed (igniter? gas? flame sensor?) so the reset attempt doesn’t just cause another lockout cycle.
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Flash codes to look up by brand
| Brand | Flash code guide |
|---|---|
| Goodman / Amana | Goodman furnace flash codes |
| Carrier / Bryant | Carrier furnace fault codes |
| Trane / American Standard | Trane furnace fault codes |
| Rheem / Ruud | Rheem/Ruud furnace flash codes |
| Lennox | Lennox furnace fault codes |
| York / Coleman / Luxaire | York/Coleman furnace flash codes |
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Symptom → most likely cause quick reference
| Furnace behavior | Most likely cause | First check |
|---|---|---|
| Completely dead (no display, no sound) | Power: breaker, emergency switch, door switch | Step 2 above |
| Thermostat calls, nothing happens | Low-voltage issue: blown board fuse, thermostat wiring | Check board fuse, flash code |
| Inducer runs, no ignition | Igniter failure or gas supply | Igniter guide + gas valve position |
| Ignites briefly then shuts off | Dirty flame sensor | Flame sensor cleaning guide |
| Runs but blows cold | Filter, high-limit tripping, cracked heat exchanger | Check filter, then call tech |
| Starts OK, shuts off quickly | Short cycling: high-limit, oversizing, or clogged filter | Replace filter, note flash code |
| Loud bang on startup | Delayed ignition: dirty burners or gas pressure | Call tech |
| Gas smell | Gas leak | Exit building, call gas company immediately |
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When to call a licensed HVAC technician
- Gas smell or CO alarm: evacuate and call immediately
- Steps 1–4 check out but furnace still won’t ignite: gas valve or control board issue
- Flame sensor cleaning doesn’t resolve ignition failure
- Recurring high-limit trips after filter is clean and airflow is confirmed
- Loud banging, booming, or screeching sounds
- Any code related to rollout or gas valve
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FAQ
My furnace is running constantly but the temperature isn’t rising. What’s wrong? Two possibilities: the furnace is undersized for current outdoor temperatures (a day with unusually cold weather can exceed design load), or heat is leaving the house faster than the furnace can add it (check doors, windows, insulation). Confirm the furnace is actually igniting (watch the startup sequence) — if it’s just blowing with no flame, see Step 4.
The furnace starts, the vents get warm, but it shuts off after 5–10 minutes. Is that normal? Not if it keeps cycling. Short cycling (starting and stopping repeatedly before reaching setpoint) is typically caused by a dirty filter (high-limit trips), an oversized furnace, or a failing thermostat. Check the filter first.
The furnace ran all night but the house is still 58°F. It’s 15°F outside. Should I call? If the furnace is running normally (igniting, blowing warm air) but the house won’t warm past 58°F in extreme cold: the furnace may be undersized for the load, or there may be significant heat loss (drafty windows, poor insulation). Before calling, confirm warm air is actually coming from the vents (not just fan air). If the air is warm and the house still can’t keep up on the coldest days, that’s a sizing or insulation issue, not a furnace problem.
Can I use space heaters while waiting for furnace repair? Yes, but safely: don’t run extension cords to space heaters, keep them away from curtains and furniture, and don’t use them while sleeping. Electric space heaters are expensive to run (roughly 10× the cost per BTU of natural gas). If you can’t fix the furnace quickly, consider a gas fireplace insert or a hotel if temperatures are dangerous.
For what a furnace repair should cost and when replacement makes more sense, see the HVAC repair and replacement cost index.
