When your furnace won’t turn on during the first cold snap, the cause is usually small. Most November no-heat calls we run in Dallas-Fort Worth come down to a thermostat setting, a tripped switch, or a filter nobody has changed since spring. Here are eight checks in the order a technician would do them, where to stop, and what the blinking light means.
Safety first. If you smell gas, do not flip switches or light anything. Leave the house and call your gas utility and 911 from outside. Everything below assumes no gas odor.
In short
- Set the thermostat to Heat, at least 3 degrees above room temperature, and wait 5 minutes.
- Check the breaker and the light-switch-style power switch next to the furnace.
- Pull the filter. A clogged one trips the high-limit switch.
- Gas smell, CO alarm, or three lockouts in a row: stop and call a pro.
Furnace won’t turn on: 8 things to check in order
Work from easiest to hardest. Give the furnace 3 to 5 minutes after each change, because control boards run a timed delay before they try to light.
1. Thermostat set to Heat and above room temperature
Still the number one cause. Confirm the mode is Heat, the setpoint is above the room reading, and the fan is on Auto. Check that a “hold” or vacation setting is not overriding your schedule. If the unit is over 10 years old, the thermostat page covers the models we install.
2. Thermostat batteries
Many thermostats run on two AA batteries. A blank or flickering screen means new batteries. If the display is fine but the furnace does nothing, see what to do if no power reaches the thermostat.
3. Breaker and the furnace power switch
Look for a breaker labeled Furnace, Air Handler, or HVAC. A tripped one often sits halfway. Push it fully Off, then On. Next, find the switch beside the furnace. It looks like a light switch, and people flip it off by accident in attics and closets. It must be On.
4. Furnace door and panel safety switch
The lower blower door presses a button that cuts power when the panel is off. If the door was not seated after a filter change, the furnace will act dead. Push it in until it clicks.
5. Clogged filter and the high-limit switch
A dirty filter chokes airflow. The furnace overheats, the high-limit switch opens, and the burners shut off while the blower keeps running. Replace filters every 1 to 3 months in heating season. If the furnace won’t turn on after a new filter, cycle the power switch off for 30 seconds to reset the limit. If it keeps tripping, read about furnace cycling frequency.
6. Condensate drain and float switch (high-efficiency units)
Condensing furnaces (90 percent AFUE and up) drain water through a PVC line, often with a float switch in the pan. A clogged line fills the pan, the float switch opens, and the furnace will not run. Clear the line and it should restart.
7. Gas supply valve and ignition
The valve on the gas pipe should be parallel to the pipe, meaning open. If other gas appliances also will not light, call the gas company. Furnaces built since the 1990s use a hot surface igniter, a ceramic element that glows orange before the gas valve opens. Igniters typically last 3 to 5 years and crack without warning. Older furnaces have a standing pilot that can blow out. Relighting a pilot is a homeowner job. Replacing an igniter is not.
8. The flashing LED error code
Look through the window in the blower door. A steady light means the board is powered. Count the flashes and match them to the sticker inside the door:
- 1 or 2 flashes: ignition failure or pressure switch.
- 3 flashes: pressure switch or inducer motor, often a blocked vent or drain.
- 4 flashes: high-limit open, usually a dirty filter.
- 5 or 6 flashes: flame or rollout fault. Stop and call.
- Rapid flashing: polarity or voltage fault at the board.
Codes vary by brand, so trust the sticker. Note the pattern before you reset, because resetting clears it.
When a furnace won’t turn on and you should call a pro
Stop and call for heating repair if you hit any of these:
- Gas smell. Leave and call the gas company first.
- Carbon monoxide alarm. Get everyone outside and call 911. The EPA explains why low CO levels still matter. Replace CO alarms every 7 to 10 years.
- Repeated lockouts. Most boards try three times, then lock out for an hour. Three lockouts in a day points to a failing igniter, flame sensor, pressure switch, or gas valve.
- Igniter or control board. These carry line voltage and gas controls. Quoted after inspection.
- Furnace age. A furnace lasts 15 to 20 years. In that range, ask for a repair-versus-replace estimate. The heating services page covers both.
Symptom, likely cause, and who fixes it
| Symptom | Likely cause | DIY or pro |
|---|---|---|
| Thermostat blank | Batteries, breaker, door switch | DIY |
| Blower runs, no heat | Dirty filter, igniter, flame sensor | Filter first, then pro |
| Clicks, no flame | Cracked igniter or gas valve closed | Check valve, then pro |
| Runs 2 minutes, stops | Dirty flame sensor, limit switch | Pro |
| Water near furnace | Condensate line, float switch | DIY, clear the line |
| 5 or more flashes | Rollout or flame fault | Pro, stop resetting |
Blower running but air cool? Read why a heater blows cold air.
The DFW first-cold-front problem
North Texas furnaces sit idle from March until the first real front, and the first freeze usually lands in late November. Months of attic dust settle on the burners. On the first run it burns off, and the house smells scorched for 20 to 30 minutes. That is normal once a season. A smell that lingers, or an electrical or rotten-egg odor, is a problem. Our furnace odor guide sorts them out. The NFPA also says keep anything flammable at least 3 feet from heating equipment, which matters in attics used for storage.
The best fix for a furnace won’t turn on call in November is a fall tune-up before the front arrives. Our HVAC maintenance visit follows the ENERGY STAR checklist: igniter, flame sensor, gas pressure, limit switches, and the drain. Club members get those visits scheduled automatically plus priority service. For what a tune-up does and does not do, see five furnace myths worth dropping.
Furnace won’t turn on FAQ
My furnace won’t turn on after a power outage. What now?
Check the breaker, then the switch at the unit, then cycle that switch off for 30 seconds and on. Outages often leave the board locked, and a power cycle clears it.
Is it safe to relight the pilot myself?
Yes, if the furnace has a standing pilot and the label instructions are intact. If it will not stay lit after two tries, call.
What temperature should I set the thermostat to?
About 68 F while you are home and awake. Setting it higher does not heat the house faster. The furnace just runs longer.
Can I reset the furnace more than once?
Once or twice is fine. Repeated resets on a furnace that keeps locking out can leave unburned gas in the chamber. After the second failure, call.
Still no heat? Call The Right Choice
The Right Choice Heating and Air repairs every furnace brand across Dallas-Fort Worth, including Dallas, Fort Worth, Arlington, Plano, Frisco, Carrollton, Farmers Branch, and Irving. If your furnace won’t turn on after these checks, call (214) 251-4015 or schedule an appointment online.