Quick Answer
When your furnace isn't working, first check: thermostat settings and batteries, air filter (clogged filters cause shutdowns), circuit breaker, and furnace power switch. If the pilot light is out (older furnaces), relight it. Many furnace problems have simple fixes. Call a pro for ignition failures, strange smells, unusual noises, or if basic troubleshooting doesn't help.
Before you call for heating repair, try these troubleshooting steps. Many furnace problems have simple solutions you can fix in minutes. We'll walk through the most common issues, starting with the easiest fixes.
Problem 1: Furnace Won't Turn On At All
If your furnace won't start, check these in order:
Basic Checks
- 1
Check thermostat
Set to HEAT, temperature above current room temp, fan on AUTO. Replace batteries if needed.
- 2
Check circuit breaker
Find the furnace breaker in your electrical panel. Reset if tripped.
- 3
Check furnace power switch
There's usually a light switch on or near the furnace. Make sure it's ON.
- 4
Check access panel
Most furnaces won't run if the access panel is loose. Push it firmly closed.
- 5
Check condensate drain (high-efficiency furnaces)
If the drain is clogged, a safety switch shuts down the furnace.
Problem 2: Furnace Runs But No Heat
Blower runs but air is cold? Check:
No Heat Checklist
- Thermostat fan set to ON instead of AUTO (blows unheated air between cycles)
- Gas valve turned off (there's usually a shutoff near the furnace)
- Pilot light out (older furnaces with standing pilot)
- Ignition failure (newer furnaces—may need professional service)
- Gas supply issue (are other gas appliances working?)
Problem 3: Dirty Air Filter (Most Common Issue)
A clogged filter restricts airflow, causing the furnace to overheat and shut down on safety limits. This is the #1 cause of furnace problems.
How to Check
Problem 4: Pilot Light Issues (Older Furnaces)
If your furnace has a standing pilot light (common in pre-2000 furnaces) and it's out:
Relighting the Pilot
- 1
Turn gas valve to OFF
Wait 5 minutes for any gas to dissipate.
- 2
Turn gas valve to PILOT
Hold down the pilot button.
- 3
Light the pilot
Use a long lighter or match to ignite the pilot while holding the button.
- 4
Hold for 30-60 seconds
This heats the thermocouple.
- 5
Release button and turn to ON
Pilot should stay lit. If not, thermocouple may need replacement.
If You Smell Gas
Problem 5: Furnace Short Cycling
Furnace runs for a few minutes then shuts off, repeatedly? Causes include:
- Dirty filter (check this first!)
- Overheating due to blocked vents
- Faulty thermostat or bad location
- Flame sensor needs cleaning
- Oversized furnace for your home
Problem 6: Furnace Making Noises
| Noise | Likely Cause | Action |
|---|---|---|
| Banging/popping | Ductwork expansion or delayed ignition | Normal duct noise is okay; delayed ignition needs service |
| Squealing/screeching | Blower motor or belt issue | Call for service |
| Rumbling | Dirty burners | Schedule cleaning |
| Clicking (repeated) | Ignition trying repeatedly | May need ignitor replacement |
| Grinding | Motor bearing failure | Turn off and call immediately |
Problem 7: Furnace Blowing Cold Air Then Hot
If your furnace blows cold air for a minute before heating, that's normal—the fan delay allows the heat exchanger to warm up. If it alternates cold/hot repeatedly, check:
- Dirty filter causing overheating cycles
- Thermostat fan set to ON (should be AUTO)
- Flame sensor may need cleaning
Problem 8: Uneven Heating
Some rooms hot, others cold? This is usually not a furnace problem but a distribution issue:
- Check that all vents are open and unblocked
- Balance dampers in ductwork (if accessible)
- Rooms far from furnace may need duct improvements
- Consider a zoning system for large homes
Problem 9: High Energy Bills
If heating bills have increased without usage changes:
- Dirty filter forcing system to work harder
- Aging furnace losing efficiency
- Duct leaks wasting heated air
- Thermostat set too high or programmed incorrectly
- Home air leaks letting heat escape
Problem 10: Yellow Pilot Flame
A healthy pilot flame is blue with a small yellow tip. A mostly yellow or orange flame indicates incomplete combustion—possibly from dirt, improper air mixture, or burner problems. This can produce carbon monoxide.
Yellow Flame Warning
When to Call a Professional
Call for Service When
- Gas smell (leave house first, call from outside)
- Carbon monoxide detector alarms
- Basic troubleshooting doesn't resolve the issue
- Pilot won't stay lit after multiple attempts
- Ignition system problems (modern furnaces)
- Strange smells (burning, electrical)
- Grinding or screeching noises
- Yellow/orange pilot flame
- Furnace keeps tripping breaker
- You're not comfortable troubleshooting further
Furnace Repair Cost Estimates
Common Furnace Repairs
| Service | Cost | Notes |
|---|---|---|
| Diagnostic | $89 | Waived with repair at Kodiak |
| Ignitor replacement | $150-$300 | Very common repair |
| Flame sensor cleaning/replacement | $100-$200 | Common issue |
| Blower motor | $400-$700 | Labor-intensive |
| Draft inducer motor | $400-$700 | Common on high-efficiency |
| Control board | $400-$800 | Electronic brains of furnace |
| Heat exchanger | $1,500-$3,000 | Major repair—consider replacement |
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Written by
Kodiak HVAC Team
HVAC professional at Kodiak Heating & Cooling.