If your furnace won't turn on, check four things before you call anyone: the thermostat settings and batteries, the air filter, the power switch next to the furnace, and the breaker. You can check all four in about ten minutes, and sometimes that's the whole fix. If they're all fine, the problem is inside the unit, and the way it fails tells you a lot about what broke.
The first cold snap in North Texas tends to expose furnaces that sat untouched since March. Here is how to work through it, in order, and what to tell the tech if you end up calling our heating team.
Start at the thermostat
A lot of no-heat problems start and end here, so rule it out first.
Make sure it's set to Heat, not Cool or Off.
Set the temperature at least three degrees above what the room reads right now.
If the screen is blank or dim, replace the batteries. Many thermostats keep running on weak batteries but stop sending the call for heat.
If you have a smart thermostat, check the app for a schedule or "eco" mode that's holding the temperature down.
If the thermostat clicks and the screen says it's heating but nothing happens at the furnace, move on.
Check the furnace switch and the breaker
Every furnace has a power switch close by. It usually looks like a regular light switch on the wall or the side of the unit, often in the attic or a hall closet. It's easy to flip off by accident, especially after someone has been up in the attic putting away holiday decorations.
Then check your electrical panel. Gas furnaces still need electricity for the blower, the igniter and the control board. If the breaker is tripped, reset it once. If it trips again, leave it off and call. A breaker that keeps tripping means something is shorting, and that's not a DIY fix.
Look at the air filter
A clogged filter does more than hurt air quality. It chokes airflow, the furnace overheats, and a safety switch (the high limit switch) shuts it down. Some furnaces will then lock out completely after a few failed tries.
Pull the filter and hold it up to a light. If you can't see light through it, replace it. Then turn the furnace off at the switch for a minute and back on to reset it.
Check the drain line on high-efficiency furnaces
If your furnace has white PVC pipes coming out of it, it's a high-efficiency unit that makes water as it runs. That water drains through a small line and sometimes a condensate pump. If the line clogs or the pump fails, a float switch stops the furnace so it doesn't flood your attic. Look for standing water in the pan or around the pump.
What the symptoms tell you
If the basic checks don't fix it, pay attention to what the furnace does when the thermostat calls for heat. This is the most useful thing you can tell a technician.
Nothing happens at all. No hum, no fan, no click. This points to power: the switch, a breaker, a blown fuse on the control board, or a failed transformer. The thermostat wiring is another possibility.
You hear a click or a hum, then nothing. The furnace is getting power and trying to start. The inducer motor (the small fan that clears exhaust before ignition) may be failing, or the pressure switch isn't confirming the vent is clear.
It tries to light, then shuts off after a few seconds. That's usually a dirty flame sensor. The burner lights, the sensor can't confirm the flame, and the furnace shuts the gas off as a safety measure. It's one of the cheaper furnace repairs. We cover it and the other common parts in our furnace repair guide.
No glow, no ignition, but the fan runs. Often a cracked hot surface igniter. If warm air never comes and the fan keeps blowing, see our post on why a furnace blows cold air.
A blinking light on the control board. Most furnaces have a small window on the lower panel where an LED blinks a code. Count the blinks and match them to the chart printed inside the panel door. Take a photo of both. It saves time when the tech arrives.
If you've worked through all of this and still have no heat, that's the point where the next step is a technician with a meter, not more guessing. You can book a furnace repair visit online and tell us what the furnace did when you tried it.
When to stop and call right away
Don't keep resetting a furnace that keeps shutting down. Call if:
You smell gas. Leave the house first and call the gas company from outside, then call us.
The breaker trips more than once.
You see water pooling around the unit.
You hear a bang or a loud rumble when it tries to light.
The furnace is over 15 years old and has needed more than one repair recently.
How to keep it from happening next winter
Most of these failures give warning signs during a fall tune-up. A dirty flame sensor gets cleaned. A weak igniter or inducer motor gets flagged before it quits on a 30-degree night. A fall furnace check catches them while they're still small, cheap fixes.
Still no heat?
Call us and tell us what you're seeing. We'll find the actual failure instead of guessing, give you a written price before any work starts, and tell you straight if the repair isn't worth it on an older unit.
Schedule furnace repair or call (469) 285-4457. Custom Cooling & Contracting is family-owned and based in Frisco, serving Dallas, Plano, McKinney, Allen, Prosper and the cities around them.
Frequently asked questions
Why is my furnace not turning on but the thermostat is on?
Check that the thermostat is set to Heat with the setpoint above room temperature, then check the furnace power switch, the breaker and the filter. If the thermostat shows it's calling for heat and the furnace still does nothing, the problem could be the thermostat wiring, a blown control board fuse or a failed part inside the furnace.
How do I reset my furnace?
Turn off the power switch next to the furnace (or the breaker), wait about a minute, and turn it back on. That clears a lockout after a few failed ignition attempts. If it locks out again, something is causing it, and resetting it over and over won't fix it.
Can a dirty filter stop a furnace from turning on?
Yes. A clogged filter restricts airflow, the furnace overheats, and the high limit switch shuts it down. Some units lock out after that happens a few times. Replace the filter, then reset the furnace.
Why does my furnace click but not turn on?
A click usually means the control board is getting the call for heat and trying to start. If nothing follows, the inducer motor, pressure switch or igniter is the likely problem. That needs a technician to test.
Is it safe to keep restarting my furnace?
Once or twice to clear a lockout is fine. If it keeps shutting down, stop. Repeated failed ignitions can mean a venting or gas problem, and the safety switches are shutting it down for a reason.