Most gas furnace problems are a safety check doing its job. A modern furnace tests something at each step of its start: that air moves through the vent, that a flame really lit, that the metal isn’t overheating. When a test fails, it stops there and often shows a code. So the useful questions are where it stopped, what the code says, and which checks are yours and which a technician’s. The guides below take each symptom in turn, every step tied to a maker’s manual or a federal agency.
If you smell gas
Leave the house now. Don’t switch lights or appliances on or off, don’t use a phone inside, and don’t try to find the leak. From outside or a neighbor’s home, call 911 or your gas utility’s emergency line, and don’t go back in until they say so (PHMSA, the federal pipeline safety agency). On propane, Trane’s owner guide warns that odorant can fade, so don’t rely on smell alone; a fuel-gas detector helps (Trane 32-5083-1C-EN, 2024).
If a carbon monoxide alarm sounds
Get everyone outside into fresh air, call 911 or the fire department from outside, and don’t go back in until responders say it’s safe. Don’t look for the source first (CPSC). Headache, dizziness, weakness, nausea, chest pain or confusion that eases away from home can be carbon monoxide (CDC). Don’t run the furnace again until a technician has checked it.
If any part of the furnace has been under water, don’t turn it on, reset it or try to light it. Carrier calls a flood-damaged furnace “extremely dangerous” and says a qualified service agency should inspect it and replace the gas controls and electrical parts that got wet (Carrier owner’s manual 80GSFRN-B-03OM, Edition 08/22, p. 2); Trane and Lennox say the same.
General information, not professional advice. Your system’s maker, your manual and a qualified technician decide what’s safe for your equipment, and local codes decide what needs a permit or a licensed installer.
We recommend no contractor, product or brand; brands appear only because their manuals are cited. The label inside your furnace door and your own manual win wherever they differ from what we write.
How a furnace starts, and why that matters
Carrier’s owner’s manual for its 80% furnaces describes a normal start (80GSFRN-B-03OM, p. 5). The inducer, a small fan motor that pulls combustion gases out through the vent, starts first. The hot surface ignitor, a ceramic part that glows orange, heats up. After about 17 seconds the gas valve opens and the burners light. After another 25 to 45 seconds the main blower starts sending warm air through the house. A pressure switch proves the inducer is moving air, and a flame sensor (a metal rod in the flame) proves the burners lit.
So the best first step costs nothing: raise the thermostat a few degrees, stand near the furnace with its doors on, and notice how far it gets. A hum with no glow, a glow with no flame, or a flame that goes out within seconds each point to a different stage. Our guide to a furnace that won’t turn on turns that into a table and walks through the makers’ owner checks in order.
Read the code before you switch anything off
If your furnace has a status light or display, read it before you open a door or cut the power. The blower door switch cuts power to the board, and on Carrier’s 59SC5 a brief power interruption ends an ignition lockout, so the code you need may vanish. On that Carrier board, you watch the status light through the blower door without taking anything off (Carrier 59SC5A-11SI, Edition 10/18, p. 75).
Codes differ by brand. “33” is a limit or rollout fault on that Carrier board and a rollout fault on Rheem’s, while Goodman shows its overheating limit as “E E 3”. Our furnace error code lookup gives each maker’s own meaning for the Carrier, Goodman, Lennox and Rheem boards we have read, says who the maker expects to act, and names the document.
Find your symptom
It blows cold air
Sometimes that is normal: with the thermostat fan set to ON, the blower runs between heating cycles and room-temperature air feels cool (Carrier 80GSFRN-B-03OM, p. 5). Otherwise the burners have usually shut off while the blower runs, most often because a dirty filter let the furnace overheat, or because it failed to keep a flame. On some newer systems paired with an R-454B or R-32 air conditioner, a refrigerant leak sensor turns the gas heat off and runs the fan on purpose; Goodman’s board shows “EAL” when that happens. Furnace blowing cold air sorts these by pattern.
It starts and stops over and over
Short cycling is usually a safety control at work. The makers’ usual reasons: too little air across the heat exchanger trips the limit switch, the control can’t prove the flame, the vent or condensate drain keeps the pressure switch from closing, or the thermostat ends each call early. If it keeps cycling on its limit switch after you’ve checked the filter and vents, Trane’s owner guide says to contact a qualified service technician “immediately”. The details are in furnace short cycling.
It’s making a noise
Trane and Lennox both say a soft hum is normal once the furnace is running; a buzz is not. Squealing usually points to the blower motor’s bearings, and whistling to choked airflow, often the filter. One sound can’t wait: a loud bang or boom as the burners light. Carrier calls it delayed ignition: shut the furnace down and call for service (80GSFRN-B-03OM, p. 7). Furnace noises goes sound by sound.
It smells
A dusty, slightly burnt smell the first few times the furnace runs each fall is usually dust burning off, and Carrier’s homeowner page puts it at about 30 minutes to a few hours. Rotten eggs or sulfur can mean gas: leave and call from outside. Hot plastic or an electrical burning smell means turn the furnace off and call a technician. Exhaust or fumes can mean combustion gases are getting in. Furnace smells covers each one.
Carbon monoxide and the heat exchanger
Carbon monoxide has no smell, so no nose can warn you of it. A gas furnace makes some CO every time it burns gas and, working as designed, vents it outdoors. CO gets in when that path fails: a blocked or disconnected vent, a leaking heat exchanger, a chimney that back-drafts, or dirty burners. CPSC advises a CO alarm certified to UL 2034 on every level of the home and outside sleeping areas. Carbon monoxide and your furnace covers alarms, symptoms and placement.
The heat exchanger is the metal chamber that keeps the flame’s exhaust apart from your house air. The Department of Energy says leaking heat exchangers “mix combustion gases with house air” (DOE Energy Saver, Internet Archive copy of 31 May 2026). There is no owner test for a crack: only a technician can confirm one. Soot or rust at the burners, or flames that aren’t bright blue, are signs Carrier sends to a technician (80GSFRN-B-03OM, p. 7). Cracked heat exchanger explains the signs, red tags, and what to ask if you’re told yours is cracked.
Your part, and where it ends
Every page here follows the same line.
- You check: thermostat, furnace switch and breaker, blower door, filter, gas valves, snow on the vent pipes, the code, and one lockout reset exactly as your own manual prints it. Makers differ here: Trane’s owner table says to turn the power on-off-on-off twice in 30 seconds, while Carrier’s owner’s manual says to shut the furnace down and call for service if lockout occurs.
- Stop: a gas smell, a CO alarm, a code that comes back after the reset, a breaker that trips again, a tripped rollout switch, a bang at lighting, soot or rust at the burners, or water on or in the furnace.
- Technician: the pressure switch, inducer, vent and drain; the ignitor and flame sensor; the gas valve and gas pressure; why a limit or rollout switch opened; the board; and the heat exchanger.
For the parts themselves, see the flame sensor and the pressure switch in our thermostats and parts section; for what heads off many of these faults, see furnace maintenance and how often to change the filter.
Before you call
Write down the code exactly as shown, the brand, model and serial number from the rating plate, what happened and when, and whether you reset it. Ask which part failed and whether it is under warranty. If the repair is large and the furnace is older, our repair-or-replace check sets your own repair quote against your own replacement quote.
What can’t wait: a gas smell, a CO alarm, a burning electrical smell, or a bang when the burners light.
Sources checked on 10 November 2026.
