When your gas furnace stops working on a cold winter night, panic can set in quickly. Before you call for emergency service, there are several troubleshooting steps you can safely perform yourself. This guide walks you through diagnosing common gas furnace problems and helps you understand when a DIY fix is possible versus when you need a professional.
Before You Start: Safety Checklist
No gas smell
If you smell rotten eggs, leave immediately
CO detector working
Test your carbon monoxide detectors
Know where gas shutoff is
Usually a valve near the furnace
Have a flashlight ready
For inspecting components
Problem 1: Gas Furnace Won't Turn On
If your furnace doesn't respond at all when you adjust the thermostat, work through these checks in order:
Step 1: Check Your Thermostat
- Is it set to "HEAT" mode? (Not "Cool" or "Off")
- Is the temperature set higher than the current room temperature?
- If it's battery-powered, replace the batteries
- Try turning the temperature up 5 degrees above current room temp
Pro Tip: Smart thermostats can sometimes lose their programming. Try a manual override to test if the furnace responds.
Step 2: Check Power Supply
- Furnace switch: Look for a light switch on or near the furnace—make sure it's ON
- Circuit breaker: Check your electrical panel for a tripped breaker labeled "Furnace" or "HVAC"
- Blown fuse: Some furnaces have a fuse on the control board
Step 3: Check Gas Supply
- Locate the gas valve on the pipe leading to your furnace
- The handle should be parallel to the pipe (open position)
- If it's perpendicular, the gas is shut off—turn it parallel to open
Warning: If you turned the gas valve and now smell gas, turn it off immediately and call your gas company.
Step 4: Check the Air Filter
A severely clogged filter can cause your furnace to overheat and shut down as a safety precaution. Pull out the filter and inspect it:
- Can you see light through it? If not, replace it immediately
- Is it covered in dust, pet hair, or debris?
- Even if it looks okay, try replacing it if it's been 3+ months
✓ If These Steps Don't Work
You likely have a failed ignitor, faulty control board, or other component requiring professional diagnosis. These are not DIY repairs—they involve electrical and gas components that require specialized training.
Problem 2: Furnace Runs But Doesn't Heat (Ignition Problems)
If your blower fan runs but the air coming out isn't warm, the burners aren't igniting. This is one of the most common gas furnace problems.
Common Causes of Gas Furnace Ignition Failure
Dirty Flame Sensor
The #1 cause. The sensor can't detect the flame, so it shuts off gas as a safety measure.
DIY Difficulty: Moderate
Failed Ignitor
Hot surface ignitors crack or wear out over time and won't glow hot enough to light gas.
Requires Professional
Gas Valve Issue
The valve may be stuck or failing, preventing gas from reaching the burners.
Requires Professional
Clogged Burners
Dust, rust, or debris blocking the burner ports prevents proper ignition.
Requires Professional
What You Can Check
- Look for error codes: Many furnaces have a small window showing LED error codes. Note the blinking pattern and look it up in your manual or online.
- Listen for ignition attempts: Do you hear clicking or see glow from the ignitor? If yes, the ignitor works but gas isn't lighting.
- Check air filter again: A clogged filter can cause overheating, which triggers the limit switch and prevents ignition.
⚠️ Don't Attempt These Repairs Yourself
- Cleaning or replacing the gas valve
- Adjusting gas pressure
- Replacing the ignitor (involves electrical connections near gas)
- Cleaning burners (requires proper disassembly and gas line handling)
These repairs involve gas and electrical components that can cause fire, explosion, or carbon monoxide poisoning if done incorrectly.
Problem 3: Pilot Light Won't Stay Lit
Note: Most modern furnaces (made after 2010) don't have standing pilot lights—they use electronic ignition. If your newer furnace has a pilot light issue, it's actually an ignitor problem (see above).
For older furnaces with standing pilot lights that won't stay lit:
Why Pilot Lights Go Out
- Thermocouple problems: This safety device detects if the pilot is lit. If dirty or faulty, it shuts off gas flow.
- Drafts: Strong drafts from nearby doors, windows, or vents can blow out the pilot.
- Gas supply issues: Low gas pressure or interrupted supply prevents consistent flame.
- Dirty pilot orifice: Dust or debris blocking the tiny pilot opening.
How to Relight a Pilot Light (If Safe)
- Turn the gas valve to "OFF" and wait 5 minutes for gas to dissipate
- Turn the valve to "PILOT"
- Press and hold the reset button while using a long lighter to ignite the pilot
- Continue holding the reset button for 30-60 seconds after the pilot lights
- Release the button—the pilot should stay lit
- Turn the valve to "ON"
If the pilot won't stay lit after 2-3 attempts, stop trying. The thermocouple likely needs replacement, which requires professional service.
Problem 4: Furnace Keeps Shutting Off (Short Cycling)
If your furnace starts up, runs for a few minutes, then shuts off before your home reaches temperature—only to restart shortly after—this is called short cycling. It's bad for your furnace and your energy bills.
Most Common Causes
Dirty Air Filter (DIY Fix)
Restricted airflow causes overheating, triggering the limit switch. Replace your filter immediately.
Blocked Vents (DIY Fix)
Make sure all supply and return vents are open and not blocked by furniture, curtains, or rugs.
Thermostat Location Issue
If thermostat is near a heat source (lamp, sunny window, vent), it may sense false warmth. Move heat sources away.
Dirty Flame Sensor (Pro Required)
The sensor doesn't detect flame properly, causing safety shutoff.
Oversized Furnace (Pro Required)
A furnace too large for your home heats too quickly and shuts off before a full cycle.
When to Call a Professional
Gas furnace repairs can be dangerous. Always call a licensed HVAC technician if:
Frequently Asked Questions
Why is my gas furnace not turning on at all?
If your gas furnace won't turn on, check: 1) Thermostat is set to heat and temperature is set higher than current room temp, 2) Circuit breaker hasn't tripped, 3) Furnace power switch is on, 4) Gas supply valve is open. If all these check out, you may have a faulty ignitor or control board requiring professional repair.
Why does my gas furnace turn on but not heat?
A furnace that runs but doesn't heat typically has ignition problems. The blower may be running but the burners aren't lighting. Common causes include a dirty flame sensor, faulty ignitor, gas valve issues, or a clogged burner. Check your air filter first—a severely clogged filter can cause overheating and safety shutoffs.
What does a yellow flame on my gas furnace mean?
A yellow or orange flame instead of blue indicates incomplete combustion and potential carbon monoxide production. This is a serious safety concern. Turn off your furnace and call a professional immediately. Causes include dirty burners, improper gas pressure, or ventilation issues.
Why does my gas furnace keep shutting off after a few minutes?
Short cycling (frequent on/off) is usually caused by: overheating from a dirty filter or blocked vents, a faulty flame sensor that can't detect the flame, thermostat issues, or an oversized furnace. This wastes energy and damages your furnace—get it diagnosed promptly.
How do I reset my gas furnace?
To reset most gas furnaces: 1) Turn off the thermostat, 2) Turn off the furnace power switch or breaker, 3) Wait 30 seconds, 4) Turn the power back on, 5) Set thermostat to heat. Some furnaces have a dedicated reset button near the motor. If the furnace won't stay on after reset, call a professional.
Why is my gas furnace making strange noises?
Banging or popping usually indicates delayed ignition (gas buildup before lighting). Squealing suggests belt or bearing issues. Rattling may be loose panels or ductwork. Rumbling after shutoff could mean a dirty burner. Any unusual sounds warrant professional inspection.
Can I troubleshoot my gas furnace myself?
Basic troubleshooting like checking thermostats, filters, breakers, and gas valves is safe for homeowners. However, never attempt to repair gas lines, ignition systems, heat exchangers, or gas valves yourself. Gas furnace repairs involving gas or electrical components should always be done by a licensed professional.
How often should I replace my gas furnace filter?
Replace standard 1-inch filters every 1-3 months, depending on usage and household factors (pets, allergies). 4-inch filters can last 6-12 months. A dirty filter is the #1 cause of furnace problems—it restricts airflow, causes overheating, and triggers safety shutoffs.