When a furnace won't turn on, the cause is usually something you can check in a few minutes: the thermostat setting, the furnace power switch, a tripped breaker, or a clogged filter. If all four check out, the problem is usually the ignition system, a flame sensor, a safety switch or the gas supply, and that work belongs to a technician. In Leander and across Central Texas, a furnace can sit unused for most of the year, so the first cold night is when these problems tend to show up.
Why won't my furnace turn on?
Carrier lists four simple causes for a furnace that is not turning on: a tripped circuit breaker, incorrect thermostat settings, a dirty air filter, or the furnace power switch turned off. The more complex causes, according to Carrier's furnace troubleshooting guidance, are a faulty ignition system, a broken flame sensor, or a disruption in the gas supply.
Rule out the simple causes first. They take minutes, they need no tools, and they account for a lot of no-heat calls on the first cold morning of the season.
What should I check on the thermostat?
The thermostat should be set to Heat, with the temperature set higher than the current room temperature. Carrier also points out that a thermostat running on batteries rather than household wiring may simply need new batteries, which can leave the screen blank and the furnace with no call for heat.
Check the fan setting too. Carrier says that if the fan will not start on On or Auto, the problem may be in the furnace controls, the thermostat, or the power supply. If the fan runs but no heat comes out, Carrier says the issue is more likely in the burner or ignition, and our post on a heater blowing cold air covers that case.
Where is the furnace power switch?
The furnace power switch is usually on the side of the furnace or on a nearby wall, and Carrier notes it often looks like an ordinary light switch. It is easy to flip off by mistake when someone is storing things in a closet or attic, so confirm it is in the On position.
Then check the electrical panel. Carrier's advice is to find the breaker labeled Furnace or Heat, and if it has tripped to the middle position, flip it to Off and then back to On. A breaker that trips again right away points to an electrical fault inside the equipment, which needs a technician.
Can a dirty filter stop a furnace from starting?
A dirty filter can shut a furnace down. Carrier explains that a clogged filter restricts airflow, the furnace overheats, and its safety controls shut the system off automatically.
The part doing that is the limit switch. Carrier describes it as a temperature sensor that starts the blower once the furnace warms up and cuts the gas valve if the inside of the furnace gets too hot. If the switch reads an overheat, or fails stuck open, the control board locks out the ignition sequence and the furnace will not start.
| What the limit switch sees | Approximate temperature | What it does |
|---|---|---|
| Furnace warmed up | About 130°F | Starts the blower |
| Cooled after the burners stop | About 100°F | Lets the blower shut off |
| Upper safety limit passed | Often about 200°F | Cuts the gas valve |
Carrier lists dirty filters, blocked return vents, closed supply registers, a failing blower motor and a dirty evaporator coil as the usual reasons a limit switch trips. Replace a dirty filter, open closed vents, and move furniture off the return grilles. Carrier is clear that a limit switch should not be bypassed, because that removes the protection against fire, a cracked heat exchanger and carbon monoxide leaks.
Is there a reset for a furnace that won't start?
Many furnaces lock themselves out after failed ignition attempts, and the lockout clears on its own. Carrier's reset guidance is to turn the furnace off, wait 5 to 10 minutes for the safety lockouts to clear, and turn it back on. If it still fails to light, call a professional.
Some furnaces have a manual-reset roll-out switch with a small red or yellow button. Carrier notes that if that switch trips repeatedly, the overheating behind it has to be found and fixed, so a switch that keeps popping is a reason to call, not to keep pressing it.
Before you call, look through the small window on the furnace door for a blinking light. Carrier says modern furnaces often flash an LED error code that can point to an ignition failure, and reading the pattern to the technician helps.
What does it mean if the furnace tries to start but won't light?
A furnace that clicks or hums but never lights usually has an ignition problem. Carrier calls a malfunctioning ignition system a primary reason a gas furnace does not turn on, and says it usually takes a qualified technician to repair.
Without a working ignitor the burners cannot light, and Carrier explains that the furnace's safety features then shut it down and stop the heating sequence. On a hot surface ignitor, the warning sign is no orange glow during startup; on a spark system it is repeated clicking. A dirty flame sensor causes a similar pattern, where the burners light and then shut off because the furnace cannot confirm the flame.
The ignitor is fragile and carries high voltage, and Carrier advises homeowners not to touch the element because oils from skin can shorten its life. Carrier's broader guidance is not to attempt repairs involving gas, electrical components or the inside of the furnace unless you are qualified. We handle ignitors, flame sensors, limit switches and control boards under heating repair.
How do I know if it is a gas supply problem?
Look at the gas valve next to the furnace without touching anything else. Carrier says the valve handle should sit parallel to the gas line when it is open. If other gas appliances in the home, such as the water heater or stove, are also not working, the problem may be with the gas service rather than the furnace.
If you smell gas, Carrier's instruction is to not operate electrical switches and not try to relight the furnace. Leave the house and call the gas utility or emergency services from a safe location.
When is a furnace that won't turn on an emergency?
A furnace that will not start is uncomfortable, but a gas smell or a carbon monoxide alarm is an emergency. The CPSC recommends carbon monoxide alarms with battery backup on every level of the home and outside sleeping areas. If one sounds, get everyone outside to fresh air, call 911, and do not go back in until responders say it is safe.
Getting it fixed
Comfort Pros is a Carrier factory authorized dealer serving Leander, Cedar Park, Georgetown, Liberty Hill and Lago Vista, licensed under TACL# TACLB116912E. If your furnace still won't turn on after the checks above, call us and we will work through the startup sequence to find the part that is stopping it before recommending any repair.
The EPA recommends having a professional inspect, clean and tune up the central heating system every year. A fall maintenance visit is the time to catch a weak ignitor or a dirty flame sensor, before the first cold night finds it.