When should you leave the house?
Then call from outsideYou smell gas
Natural gas is odorized so you'll notice it. A faint whiff at startup for a second can be normal; a persistent smell is not.
- Don't touch light switches, the thermostat, or anything electrical.
- Don't hunt for the leak.
- Everyone out, pets included, door open behind you.
- From outside: your gas utility's emergency line — CenterPoint in Houston, Southwest Gas in Phoenix, Atmos in Dallas, FPL/TECO in South Florida, SoCalGas in LA — or 911.
- Don't go back in until it's cleared.
Carbon monoxide alarm sounding
CO is odorless. A cracked heat exchanger, blocked flue, or backdrafting appliance puts combustion gases into living space. Headache, nausea, confusion in more than one person at once is the other sign.
- Everyone outside immediately. Don't stop to open windows.
- 911 from outside.
- Anyone with symptoms gets medical attention — CO effects can worsen after exposure ends.
- The furnace stays off until the exchanger and venting are tested.
Hissing at a gas connection, or flame outside the burner box
Same response. Out first, call second.
After it's safe: (855) 644-0803Which furnace faults mean shut it off and call today?
Not an evacuation. Not something to run overnight.Signs of a cracked heat exchanger
Soot around the burner compartment, a flame that wavers or yellows when the blower starts, water at the base of a high-efficiency unit that isn't condensate, a CO detector that has chirped low readings. Off until inspected — with a combustion analyzer, not a flashlight.
Repeated lockouts
Tries to light, fails, retries, locks out for an hour. Three to five attempts on modern boards. A safety is working — pressure switch, rollout, limit. Find the cause; don't keep resetting.
Rollout switch tripped
Flame escaped the burner box. Manual reset on most furnaces. Resetting it without finding out why — blocked exchanger, blocked flue, low gas pressure — is how the tier above happens.
A burning smell that isn't first-of-season dust
Dust burn-off smells for twenty minutes on the first cold day and stops. Persistent, or plastic/electrical smelling, is a motor, a wire, or a board cooking.
Which heating faults can wait until this week?
Real faults, no safety indicatorsNo heat, none of the signs above
Four things fix more no-heat calls than anything else: thermostat on HEAT above room temp; thermostat batteries; the furnace switch (looks like a light switch, on or near the unit) actually on; filter not packed solid — a blocked filter overheats the exchanger and trips the limit, which reads as "no heat."
Ignition failure
Inducer runs, click, no flame — or flame lights then dies in seconds. Cracked hot-surface ignitor, dirty flame sensor, or gas valve not opening. Ignitor and sensor are wear items; techs carry both.
Short cycling
Heats a few minutes, quits before the house is warm, repeats. Limit switch tripping on overheat — filter, blocked return, failing blower. Occasionally an oversized furnace, which no repair fixes.
Blower runs, air is cold
Burner isn't. Ignition, board calling fan without heat, thermostat wiring — or the fan running to clear the exchanger during a lockout.
Squeal, grind, or a thump on startup
Squeal/grind is a blower or inducer bearing. Thump at ignition is delayed ignition from dirty burners. Neither is dangerous today; both get worse.
What should a furnace tune-up include before the season?
What a tune-up actually isWorth paying for when it includes combustion analysis and a heat exchanger inspection. A filter swap and a visual for $59 is a sales visit. A real one: burner cleaning and flame inspection; heat exchanger inspection — combustion analysis on anything over ten years; flame sensor cleaning and ignitor resistance; inducer and blower amp draws, capacitor test; manifold gas pressure against the rating plate; limit, rollout, pressure switch operation; flue and venting; condensate trap on high-efficiency units — a clogged trap is a top cause of pressure-switch lockouts. $100–$250, an hour or more.
Winter in these five metros is short, which is exactly why furnaces here fail on the first cold snap — they sat eight months. Get it looked at in October. The maintenance page covers plans.
Combustion side
- Gas valve opens on the board's command
- Burners fire into the sealed exchanger cells
- Exchanger cells carry the flame gases up and over
- Inducer pulls those gases through the cells
- Flue takes them outside — and nowhere else
House-air side
- Return air comes back from the rooms
- Blower pushes it across the outside of the cells
- Heat crosses the metal, gases do not
- Supply air goes out to the rooms, warm and clean
What does each furnace component do, and how does it fail?
What each part does, how it fails, what replacement typically runs in these markets.
| Component | What it does | How it fails | Typical |
|---|---|---|---|
| Hot surface ignitor | Ceramic element that glows to light the gas. | Cracks from thermal cycling. Sequence runs, valve opens, nothing lights. The most common furnace repair there is. | $150–$350 |
| Flame sensor | Rod that proves flame so the valve stays open. | Coats with oxide. Lights, dies in 3–5 s. Cleaning usually fixes it. | $80–$250 |
| Inducer motor | Pulls combustion gases through the exchanger and out the flue before ignition. | Bearings or a cracked wheel. Won't start its sequence; pressure switch never closes. | $400–$900 |
| Pressure switch | Proves the inducer is moving air. | Often fine — the condensate trap or flue is blocked upstream. | $150–$350 |
| Limit switch | Shuts the burner off if the exchanger overheats. | Usually trips because of airflow, not because it's broken. | $150–$300 |
| Blower motor | Moves air across the exchanger and through ducts. | Bearings, capacitor, or the ECM control module. Squeal, then no airflow, then the limit trips. | $400–$1,200 |
| Control board | Sequences inducer, ignitor, valve, blower. | Relay failure or a blown trace. Erratic symptoms. Diagnosed by outputs, not by swapping. | $400–$1,000 |
| Gas valve | Opens on command to feed the burners. | Solenoid fails or sticks. Ignitor glows, no gas. Licensed tech only. | $300–$700 |
| Heat exchanger | Separates combustion gases from the air you breathe. | Cracks with age. The CO risk. Past 12–15 years it's a new-furnace conversation. | $1,500–$3,500 or replace |
Heating repair is a service call like any other: describe the symptom, get the repair quoted, and let a licensed heating technician do the work. The checklist below is what to have ready.
Before you call for heating repair
- Thermostat on HEAT above room temperature, and not on EMERGENCY HEAT on a heat pump
- Furnace switch and breaker on — reset the breaker once, never repeatedly
- Pilot or ignitor: a standing pilot that will not stay lit is a thermocouple; a click with no flame is the ignitor or flame sensor
- Emergency or not: a gas smell or a CO alarm means outside first, then the gas utility or 911; a no-heat call with none of those is a same-day repair
- What it costs: ask for the diagnostic price and whether an after-hours emergency premium applies tonight
Furnace repair questions, answered
Is emergency furnace repair available at night?
Yes. Urgent HVAC Service is answered 24 hours a day and routes to heating contractors who run after-hours service. If you smell gas or a carbon monoxide alarm is sounding, leave the house first and call the gas utility or 911 from outside; the repair call comes after.
How much does furnace repair cost?
A flame sensor cleaning is $80–$260, a hot surface ignitor $140–$390, an inducer or blower motor $380–$1,400, a control board $380–$1,100. A cracked heat exchanger on a furnace past 12–15 years is usually a replacement conversation rather than a repair.
Why does my furnace light and then shut off after a few seconds?
A dirty flame sensor, almost always. The rod that proves the flame is coated with oxide, the board thinks the flame failed and closes the gas valve. Three attempts and it locks out. Cleaning fixes most of them.
What does a carbon monoxide alarm mean?
That combustion gas is reaching the living space, usually through a cracked heat exchanger or a blocked flue. Everyone outside immediately, then 911. The furnace stays off until the exchanger and venting are tested with a combustion analyzer.
Does a heat pump count as furnace repair?
Yes. In Miami and much of Phoenix the heating is a heat pump or electric strips, not a gas furnace, and the same line handles it. Say which system you have when you call.
Do I need a permit for a furnace repair?
No. Replacing the furnace does need a mechanical permit in Houston, Dallas, Miami-Dade and Los Angeles, and gas piping or vent changes go on the same permit. Phoenix exempts a like-for-like change-out.
How quickly can a heating technician come out?
Same-day in most cases. On the first cold front of the season every truck in the metro is out, so call as early in the day as you can.
What can I check before calling?
Thermostat on HEAT above room temperature, the furnace switch on, the filter not packed solid, the breaker not tripped. On a high-efficiency furnace, that the outside intake and exhaust pipes are clear. Those four clear a third of no-heat calls.
Furnace repair by metro
Each city page covers local climate, permits, utility rebates and the failure modes specific to that market. Each metro page covers emergency heating repair service, same-day furnace repair by a licensed HVAC technician, the ignition faults common there, and the repair cost to expect.
References: US Consumer Product Safety Commission (carbon monoxide safety) · US Department of Energy · Lennox support · Goodman support
Ask before you call
Not sure what's wrong, or whether it's urgent? Describe it. Questions that come up often get added to this page.