On for a few minutes, off, then on again. Restricted airflow, a dirty flame sensor, and a cracked heat exchanger all produce this pattern.
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Furnace problems
A furnace usually fails in one of a handful of recognizable patterns, and most of them have a specific, checkable cause before you need to call anyone.
Reading a furnace by what it does, not just that it's "broken"
"My furnace isn't working" covers a lot of different actual situations: one that does nothing at all, one that starts and stops every few minutes. And one that runs but never gets the house warm are three different problems with three different likely causes. This section works from the symptom, because that is usually the fastest way to narrow down what is actually happening before a technician even arrives.
Gas furnaces are combustion equipment, which is why several of the checks that matter most (the heat exchanger, the gas valve, the ignition system) are deliberately left to a licensed HVAC technician rather than written up as steps to do yourself. That line is drawn on purpose throughout this section.
Furnace guides
No hum, no click, nothing. A working order for checking power, the thermostat, and the gas valve before you call anyone.
Air is moving but it isn't warm. Sometimes that's a fan setting; sometimes it's ignition failing partway through a cycle.
A dusty smell on the first cold day is normal. A sharp electrical smell, and a rotten-egg smell, are not the same problem at all.
Lights, then goes out again in seconds. On most standing-pilot furnaces, one small safety part explains the pattern.
No smell, no color, no taste. What raises the risk, and the one response that's always right if a detector sounds.
The mirror image of short cycling. A stuck thermostat fan switch, a shorted wire, or a welded relay can keep it going long after it should stop.
The burner fires but the vents stay quiet. Almost always the blower motor, its capacitor, a belt, or a relay, not the burner itself.
A breaker that won't stay reset is protecting you from a real fault. Usually a failing motor or a short, not something to keep resetting past.
A rotten egg smell is never a furnace troubleshooting question. One response, and it never changes: leave and call your gas utility or 911.
Air in at the bottom, heat out the top. Why your duct layout decides the type, and what you can look after yourself.
Burners, heat exchanger, blower, flame sensor and control board, in the order they work and with the DIY line marked.
Weak airflow, squealing, early shutoffs: what each symptom points to and the short list of checks that are safe.
Why the burners shut off while the fan keeps running, and why you never bypass the switch.
How the two designs differ in venting and condensate, and which fits your climate.
Where to find the blinking light, how to read the pattern, and which checks are safe before you call.
The gas-smell check, the general steps for a standing pilot, and when to stop and call a technician.
What dirty burners look like, why cleaning them is a technician's job, and the safe upkeep that is yours.
The safe order for switching a furnace off, when to leave it running, and why the gas valve stays alone.
Why a high-efficiency furnace makes water, where it drains, and the part that stays with a technician.
Condensate, a humidifier or the AC coil above it. How to tell which one, and the part that stays with a technician.
Low heat most days, high only when needed. What that changes for comfort and noise, and when it adds nothing.
The signs that point to a crack, why they matter for carbon monoxide, and why the furnace stays off until a technician tests it.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- ENERGY STAR. Furnaces
- CDC. Carbon Monoxide Poisoning Basics