Your furnace isn't working. The house is getting cold. Before you pay a service call fee — which runs $75 to $200 in 2026 before the tech touches anything — there are several things you can check yourself. Some of them take two minutes and cost nothing. Others will tell you exactly what to tell the technician when you do call.
Work through this list in order. The most common causes are at the top.
Step 1: Check the Thermostat
Start here. Every time.
Confirm the thermostat is set to HEAT, not COOL or FAN ONLY. Confirm the set temperature is at least 3 to 5 degrees above the current room temperature — the system won't call for heat if the room is already at setpoint.
If the display is blank or unresponsive, replace the batteries. Most thermostats use AA or AAA batteries, and a dead battery is exactly this kind of problem. If the thermostat is hardwired, check that the common (C) wire connection is secure at the thermostat base.
One more thing: if the fan is set to ON rather than AUTO, it will blow room-temperature air continuously even when the furnace isn't running. That air doesn't feel like heat. Flip the fan setting to AUTO.
If you have a smart thermostat (Nest, Ecobee, Honeywell Home), check the app — sometimes the schedule has been changed or the system was manually overridden to OFF.
Step 2: Check the Air Filter
A clogged air filter is the most common reason a furnace shuts down unexpectedly. Here's why: when the filter is packed with dust and debris, airflow across the heat exchanger drops. The heat exchanger gets hot enough to trigger the high-limit safety switch, which shuts the furnace off to prevent overheating. You may hear the furnace start, run briefly, and shut down repeatedly — that's the limit switch cycling.
Pull out the filter and hold it up to a light source. If you can't see light through it, replace it now. Standard 1-inch filters should be replaced every 30 to 90 days. Thicker 4- to 5-inch media filters can last 6 to 12 months, but they're not immune to clogging, especially in homes with pets.
A new filter costs $5 to $25. Do this before anything else.
Step 3: Check the Furnace Power Switch
There's an on/off switch on or near almost every furnace — it looks identical to a standard light switch. It's usually on the wall adjacent to the unit or on the unit itself. Electricians sometimes flip these switches accidentally during unrelated work, and they get bumped off occasionally on their own.
Make sure the switch is in the ON position.
Then check the circuit breaker. Gas furnaces still require electricity to run the blower motor, control board, and ignition system. Find the furnace breaker in your electrical panel (it may be labeled "furnace," "air handler," or "FAU"). If it's tripped, flip it fully to OFF first, then back to ON.
If the breaker trips again within an hour, stop resetting it and call a technician. A repeatedly tripping breaker means there's an electrical fault in the system — continuing to reset it can damage the equipment.
Step 4: Check the Furnace Door Panel
This one surprises a lot of homeowners. Modern furnaces have a safety interlock switch built into the front access panel. If the door isn't fully latched, the switch stays open and the furnace won't run at all. You might hear the thermostat click on but nothing happens.
Remove the panel, look inside briefly, then reseat it firmly until you feel or hear it click into place. Try the thermostat again.
Step 5: Check the Pilot Light or Ignitor
How you check this depends on your furnace's age.
If your furnace is older (pre-2000): Check the pilot light
Furnaces made before roughly 2000 often use a standing pilot — a small flame that burns continuously to light the burners when the system calls for heat. If the pilot is out, the furnace won't start.
Look for a small viewport or observation window on the furnace body. If you can see the burner area and there's no flame visible, the pilot is out. Follow the manufacturer's instructions printed on the furnace door to relight it — the sequence is typically: turn the gas valve knob to PILOT, press and hold it, hold a lighter near the pilot opening, and hold the knob for 30 to 60 seconds after the flame catches to let the thermocouple heat up.
If the pilot relights but goes out immediately when you release the knob, the thermocouple is likely faulty — a $20 part that a technician can replace in about an hour.
If your furnace is newer (post-2000): Check the ignitor
Most furnaces built after 2000 use a hot surface ignitor — a silicon carbide or silicon nitride element that glows orange-hot to ignite the burners. You can usually see it through the observation window. When the furnace runs a startup cycle, you should see it glow within 30 to 60 seconds.
If the ignitor doesn't glow, it may have failed. Hot surface ignitors are fragile — even touching one with bare hands can shorten its life. Don't attempt to handle it without knowing what you're doing. Replacement is a $150 to $350 repair that most technicians can complete in under an hour.
Step 6: Check the Gas Supply
If your furnace starts a cycle, the ignitor glows, but the burners won't light — the gas supply is worth checking.
First: are other gas appliances in the house working? Fire up a gas burner on the range. If the stove works, gas is reaching the house. The issue is local to the furnace — likely the gas valve or control board.
If no gas appliances are working, check whether the main gas shutoff valve to the house is in the OPEN position. The valve is open when the handle is parallel to the pipe. If it's perpendicular, it's closed. Check whether your gas has been shut off by the utility — this happens occasionally after billing issues or in some emergencies.
One more check: look at the manual shutoff valve on the gas line going directly to the furnace. It should be parallel to the pipe (open). If someone shut it off during other work and didn't reopen it, this is your culprit.
If gas supply appears fine but burners still won't light, that's a gas valve or control board issue. This requires a technician.
Step 7: Check the Condensate Drain (High-Efficiency Furnaces)
If you have a high-efficiency furnace — any unit with an AFUE rating of 90% or above — it produces condensate as a byproduct of combustion. That moisture drains out through a PVC drain line. When the drain line clogs, the furnace triggers a safety switch and shuts down.
Look for a white PVC pipe running from the furnace to a floor drain or utility sink. Check whether it's visibly kinked, frozen, or disconnected. On some units, a small reservoir or trap in the condensate system will trip a float switch when it fills up.
A clogged condensate drain is often DIY-accessible: clear the line with a wet/dry vacuum or flush it with diluted white vinegar. If the furnace fires up after that, you've found your problem.
Step 8: Read the Error Codes
Most furnaces built after the mid-1990s have a diagnostic LED on the control board that blinks a pattern when there's a fault. The pattern corresponds to a specific error code — and there's a chart on the inside of the furnace door panel that decodes them.
Pull off the access panel and look for the blinking light. Count the blinks carefully. Common patterns across most brands:
1 blink: Ignition failure or flame sensor issue
2 blinks: Pressure switch fault (can indicate a blocked flue, failed inducer motor, or cracked pressure switch hose)
3 blinks: Limit switch open — often caused by a clogged filter, blocked vents, or overheating
4 blinks: High-limit switch fault — same causes as 3 blinks, often more severe
5 blinks: Flame sensed without call for heat — this is a gas valve leak, shut the system down and call immediately
The codes aren't universal — a 3-blink pattern means different things on a Carrier versus a Lennox versus a Rheem. But the chart on your furnace door is specific to your unit. If the door chart is missing or unreadable, search online for your furnace's model number plus "error codes."
Some codes you can address yourself (replace the filter for a limit switch fault, clear the condensate drain for a pressure switch fault). Others require a technician.
When to Stop and Call a Technician
Stop troubleshooting and call for service if:
- You smell gas. Leave the house, don't touch any switches, and call your gas utility and 911 from outside.
- The furnace is making banging, grinding, or screeching noises
- The error code indicates a cracked heat exchanger or limit switch fault that returns after you've replaced the filter
- The flame sensor light (5 blinks) indicates gas flowing without ignition — gas is leaking past the valve
- You've worked through all the steps above and the furnace still won't start
A heat exchanger issue is particularly serious. The heat exchanger separates combustion gases — including carbon monoxide — from the air circulating through your home. A cracked heat exchanger is not a "run until spring" situation. Carbon monoxide detectors are essential in any home with a gas furnace; if yours is more than 7 years old, replace it.
What to Tell the Technician
When you call, have this information ready:
- Furnace brand and model number (on a label inside the access panel)
- Age of the furnace
- What the furnace is doing — or not doing — when it tries to start
- Which error code is showing, if any
- What you've already checked
Telling the tech "it's showing 3 blinks, I replaced the filter and cleared the vents, and it still trips" gets you a much more focused service call than "it just stopped working." That clarity saves diagnostic time and can reduce your labor bill.
If you need a licensed HVAC technician in your area, post a service request on HVACJobs.IO. Describe the issue and your furnace's age and local techs will respond with availability and pricing.
Frequently Asked Questions
Why is my furnace not turning on?
The most common causes, in order of frequency: tripped circuit breaker, dirty air filter triggering the high-limit safety switch, failed ignitor, or the thermostat settings. Work through the checklist above before calling for service — a significant percentage of furnace "failures" resolve with a filter replacement or a breaker reset.
What do furnace error codes mean?
Furnace error codes are blinking LED patterns on the control board that indicate which safety switch or component has triggered a fault. The specific pattern is decoded on a chart affixed to the inside of the access panel door. Common codes point to pressure switch faults, limit switch overheating, ignition failures, or flame sensor issues. The chart on your furnace door is the authoritative reference for your specific unit.
Can I relight my furnace pilot light myself?
Yes, on furnaces with a standing pilot. The procedure is printed on the furnace door. The basic sequence: turn the gas valve to PILOT, press and hold, light the pilot, hold for 30 to 60 seconds, then release and turn to the ON position. If the pilot won't stay lit when you release the valve knob, the thermocouple needs replacement — that's a technician job.
Why does my furnace turn on and then immediately shut off?
Short cycling like this usually indicates one of three things: a clogged air filter causing the high-limit switch to trip, a dirty flame sensor that can't confirm ignition, or a pressure switch fault from a blocked flue or failed inducer motor. Replace the filter first. If the problem continues, you'll need a technician with a combustion analyzer to diagnose the pressure or flame sensor issue.
How much does it cost to fix a furnace that won't turn on?
It depends entirely on the cause. A new ignitor is $150 to $350 installed. A flame sensor cleaning or thermocouple replacement is $100 to $200. A control board is $300 to $900. A cracked heat exchanger on an older furnace often makes replacement more economical than repair — full system replacement runs $2,500 to $5,500 for a mid-efficiency gas furnace installed.
When should I replace my furnace instead of repairing it?
If the furnace is over 15 to 20 years old and facing a repair costing more than $500 to $700, run the math on replacement. A new 80% AFUE gas furnace installed runs $2,500 to $4,500; a 96% AFUE unit runs $3,500 to $5,500. The energy savings from a newer, more efficient unit combined with avoiding further repairs on aging equipment often makes replacement the smarter call.
Is it dangerous to run a furnace that's not working right?
Depends on the symptom. A furnace that won't start poses no danger. A furnace with a cracked heat exchanger, a gas smell, or a flame without ignition (5-blink code) requires immediate shutdown. Carbon monoxide is colorless and odorless — every home with a gas furnace should have a working CO detector on every floor.