Tony here.
Let’s clear something up right now:
Most startup problems are not “mysteries.” They’re unfinished work.
When a furnace fails at startup, it’s usually not broken. It’s misfed, miswired, mis-vented, or misconfigured. This guide is for beginners, homeowners, and new techs who want straight answers, not guesswork.
If your furnace won’t fire, won’t stay running, or sounds like it’s fighting itself — this is where you start.
100,000 BTU 96% AFUE Upflow/Horizontal Two Stage Goodman Gas Furnace - GR9T961004CN
🚨 Rule #1: Read the Furnace, Not Your Emotions
Before tools, before forums, before panic — observe.
Ask yourself:
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Does it try to start?
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Does it shut down immediately or after running?
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Is there an error code?
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Is anything obviously unsafe?
Tony rule:
A furnace always tells you what’s wrong. You just have to listen instead of guessing.
🔌 Problem #1: Furnace Won’t Power On at All
What it looks like:
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No lights
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No inducer
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No response to thermostat
Quick fixes:
✔ Check breaker and service switch
✔ Confirm door switch is engaged
✔ Verify transformer output (24V)
✔ Confirm polarity (hot/neutral not reversed)
Tony diagnosis:
This isn’t a furnace problem — it’s an electrical problem.
👉 Electrical basics reference:
https://www.energy.gov/energysaver/furnaces-and-boilers
🌬️ Problem #2: Inducer Runs, But Furnace Won’t Ignite
What it looks like:
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Inducer motor starts
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Igniter never glows
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Furnace times out
Likely causes:
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Pressure switch not closing
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Blocked or mis-sloped vent
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Condensate trap not filled
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Intake/exhaust reversed
Quick fixes:
✔ Check vent slope back to furnace
✔ Clear condensate drain and trap
✔ Inspect pressure switch tubing
✔ Verify intake and exhaust piping
Tony truth:
A pressure switch failure is usually an airflow problem pretending to be a switch problem.
🔥 Problem #3: Igniter Glows, But No Flame
What it looks like:
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Igniter heats
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Gas valve clicks
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No ignition
Likely causes:
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Gas shutoff closed
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Gas pressure too low
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Wrong gas type
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Dirty burners
Quick fixes:
✔ Confirm gas valve is fully open
✔ Verify gas pressure with manometer
✔ Check for propane/natural gas mismatch
✔ Clean burners if contaminated
👉 Goodman gas furnace reference:
https://www.goodmanmfg.com/products/gas-furnaces
If gas isn’t present, ignition can’t happen — simple as that.
🔥 Problem #4: Flame Lights, Then Immediately Shuts Off
What it looks like:
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Burner lights
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Shuts down after 2–5 seconds
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Repeats cycle
Likely cause:
Dirty flame sensor (almost every time)
Quick fix:
✔ Remove flame sensor
✔ Lightly clean with fine steel wool
✔ Reinstall and retry
Tony rule:
Flame sensors don’t fail — they get dirty.
👉 Combustion & flame safety (EPA):
https://www.epa.gov/indoor-air-quality-iaq/carbon-monoxides-impact-indoor-air-quality
🌡️ Problem #5: Furnace Fires but Shuts Down After a Few Minutes
What it looks like:
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Runs briefly
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Shuts off
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Repeats cycle
Likely cause:
Overheating due to airflow restriction
Quick fixes:
✔ Replace clogged filter
✔ Check return duct size
✔ Open closed supply registers
✔ Verify blower speed settings
If temperature rise exceeds spec, the furnace will protect itself — every time.
🌬️ Problem #6: Loud Airflow, Whistling, or Rattling at Startup
What it looks like:
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High-pitched noise
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Rattling ductwork
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“Jet engine” sound
Likely causes:
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Undersized return
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Excessive static pressure
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Incorrect blower speed
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Loose panels or transitions
Quick fixes:
✔ Measure static pressure
✔ Reduce blower speed if needed
✔ Seal duct leaks
✔ Secure panels and duct connections
Tony truth:
Noise is airflow trying to escape where it shouldn’t.
🧭 Problem #7: Two-Stage Furnace Always Runs in High Stage
What it looks like:
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Full blast every cycle
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Short run times
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Higher gas usage
Likely causes:
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Thermostat misconfigured
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W1/W2 wired incorrectly
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Staging timer disabled
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Dip switches set wrong
Quick fixes:
✔ Verify thermostat staging settings
✔ Check W1/W2 wiring
✔ Review control board configuration
Low stage should be doing most of the work.
💧 Problem #8: Furnace Locks Out After a Day or Two
What it looks like:
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Works initially
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Locks out later
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Reset temporarily fixes it
Likely causes:
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Condensate drain clog
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Frozen drain line
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Improper vent slope
Quick fixes:
✔ Clear and flush drain
✔ Add heat tape if needed
✔ Verify continuous slope
✔ Check drain trap depth
Tony rule:
Water doesn’t disappear. If it can’t drain, it will shut you down.
🧪 Problem #9: Weird Smells or Headaches at Startup
What it looks like:
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Chemical smell
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Headaches or dizziness
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Eye irritation
Possible causes:
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Combustion air issues
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Venting leaks
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Incomplete combustion
Immediate actions:
✔ Shut system down
✔ Ventilate space
✔ Test for CO
✔ Inspect vent connections
👉 CO safety reference:
https://www.cdc.gov/carbon-monoxide/about/index.html
Never ignore symptoms. Ever.
📋 Tony’s 10-Minute Startup Troubleshooting Checklist
Before you call it “broken,” check these:
✔ Power & polarity
✔ Gas pressure
✔ Vent slope & termination
✔ Condensate flow
✔ Flame quality
✔ Blower speed
✔ Filter & returns
✔ Thermostat staging
✔ Error codes
✔ Temperature rise
If you can’t check these, you’re guessing — not troubleshooting.
🚫 Common Rookie Mistakes That Cause Startup Failures
❌ Assuming new equipment is defective
❌ Ignoring airflow
❌ Skipping pressure measurements
❌ Overriding safeties
❌ Leaving before full cycle completes
Tony truth:
Safeties don’t cause problems. They reveal them.
🔚 Final Tony Takeaway
Startup problems aren’t personal.
They’re mechanical.
If you slow down, observe the sequence, and verify the basics, 90% of startup failures become easy fixes.
The furnace isn’t fragile.
It’s just intolerant of shortcuts.
Fix the system, not the symptom — and the furnace will take care of the rest.
