Troubleshooting Guide: Furnace Won’t Start? Here’s Your First 10 Check

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What to do before you call for service—and how to know when it’s time to stop

When a furnace won’t start, most homeowners jump straight to worst-case thinking:
“Is it dead?” “Is this going to be expensive?” “Did I ignore something?”

Take a breath.

In my experience, most no-heat calls come down to a small, simple issue—something you can safely check in minutes without tools or training.

80,000 BTU 96% AFUE Upflow/Horizontal Single Stage Goodman Gas Furnace - GR9S960803BN

This guide walks you through the first 10 checks I’d do in order, exactly the way I’d talk a homeowner through it on the phone.

🧠 Before You Start: Two Important Rules

Do not disassemble sealed components
Do not touch gas lines or valves

This guide focuses on safe checks only—observation, verification, and basic resets. If something feels off, stop.

🔢 Check #1: Thermostat Settings (It’s #1 for a Reason)

I know—it sounds obvious. But this is the most common cause.

✔️ What to Check

Set to HEAT, not COOL or OFF
Temperature set at least 5°F above room temp
Batteries replaced (if applicable)
Screen is on and responsive

Loose batteries or wrong modes stop furnaces every day.

🔌 Check #2: Furnace Power Switch

Furnaces have a dedicated service switch, usually mounted nearby and often mistaken for a light switch.

✔️ What to Check

Switch is fully ON
No signs it was bumped accidentally

I’ve fixed dozens of “dead furnaces” with this one flip.

⚡ Check #3: Circuit Breaker or Fuse

Even if nothing else in the house lost power, the furnace breaker could have tripped.

✔️ What to Do

Locate your electrical panel
Reset the furnace breaker once
If it trips again immediately—stop and call a pro

Repeated trips mean an electrical issue.

🧼 Check #4: Furnace Filter (Yes—It Can Stop the Furnace)

Modern furnaces have safety switches that shut the system down if airflow is restricted.

✔️ What to Check

Remove the filter
If it’s dirty or clogged, replace it
Make sure it’s installed in the correct direction

According to the U.S. Department of Energy, dirty filters are a top cause of HVAC shutdowns and inefficiency.

https://www.energy.gov/energysaver/home-heating-systems

🚪 Check #5: Access Panels & Safety Switches

Furnaces won’t run if access panels aren’t seated correctly.

✔️ What to Look For

Upper and lower panels fully installed
No gaps or loose corners
Panels not warped or bent

Even a slightly open panel can trip a safety switch.

🔥 Check #6: Is the Furnace Trying to Start?

Stand near the furnace and listen.

✔️ Listen For

Clicking
A brief fan start
Ignition attempt

❌ Red Flags

Complete silence
Rapid clicking with no ignition
Starts then shuts off quickly

This tells you where the failure is happening.

🧪 Check #7: Gas Supply (Observation Only)

Do not adjust valves—just observe.

✔️ What to Check

Gas appliances like stoves working?
Furnace gas valve handle parallel to the pipe?
No gas smell

If you smell gas: leave the area immediately and contact your utility.

🌬️ Check #8: Intake & Exhaust Pipes (High-Efficiency Furnaces)

High-efficiency furnaces use PVC pipes for air intake and exhaust.

✔️ What to Check

No snow, leaves, or debris blocking pipes
Pipes not sagging or disconnected
Outdoor termination clear

Blocked exhaust = furnace lockout.

The EPA confirms blocked venting is a common cause of combustion appliance shutdowns.

https://www.epa.gov/indoor-air-quality-iaq/what-are-combustion-products

🔁 Check #9: Furnace Reset (Once Only)

Some furnaces allow a basic reset.

✔️ How to Do It

Turn furnace power OFF
Wait 60 seconds
Turn power back ON
Observe startup sequence

If it fails again—don’t keep resetting.

Repeated resets can mask real problems.

🚨 Check #10: Error Codes or Warning Lights

Most modern furnaces have a diagnostic LED.

✔️ What to Do

Look through the sight glass
Count blinking lights
Match pattern to the panel diagram

This gives technicians a huge head start—and tells you if it’s DIY or not.

Manufacturer diagnostic basics (Goodman example):
🔗 https://www.goodmanmfg.com/resources/hvac-learning-center

❌ When to STOP Troubleshooting Immediately

Call a professional if you notice:

Gas smells
Burnt electrical odors
Loud banging or grinding
Furnace repeatedly short-cycling
Error codes related to ignition, flame, or pressure switches

These are not homeowner fixes.

💰 What These 10 Checks Can Save You

In real-world service calls:

50–60% are resolved by checks #1–#5
Another 20% involve filters or airflow
Only the rest require parts or repair

That’s time, money, and stress saved.

🧾 Tony’s Straight Advice

Here’s how I approach it:

Check the simple stuff first
Never guess with gas or flame
Don’t override safety features
Know when to stop

Troubleshooting is about elimination, not heroics.

🏁 Final Word from Tony

A furnace that won’t start doesn’t automatically mean a big repair. Most of the time, it’s a setting, a switch, a filter, or a safety condition doing its job.

Run these 10 checks calmly and in order.
If it still won’t start—you’ll know you did your part before making the call.

That’s smart homeownership.