When a Goodman furnace flashes a code, most homeowners do one of two things:
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Panic
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Ignore it and hope the heat stays on
Both are mistakes.
100,000 BTU 96% AFUE Upflow/Horizontal Two Stage Goodman Gas Furnace - GR9T961004CN
Those blinking lights aren’t random. They’re your furnace telling you exactly what it’s unhappy about — sometimes before it actually shuts down.
This guide explains:
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What Goodman diagnostic codes really are
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Why codes matter even if the furnace is still running
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The most common Goodman furnace codes (in plain English)
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What you can safely check yourself
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When a code means “stop and call a pro”
No memorization required. Just understanding.
🔌 What the Goodman Furnace Control Board Actually Does
Think of the control board as the furnace’s brain.
It:
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Manages ignition timing
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Controls the blower motor
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Monitors safety switches
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Records faults and shutdowns
When something falls outside safe limits, the board responds in two ways:
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Protects the furnace
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Reports the issue through diagnostic codes
Those flashes are not failures — they’re warnings or confirmations of protection.
💡 How Goodman Diagnostic Codes Work
Most Goodman furnaces use:
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A single LED light
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A flash pattern
The pattern usually looks like:
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Short flashes
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A pause
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Repeating sequence
Example:
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Flash–flash–pause = Code 2
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Flash–flash–flash–pause = Code 3
The code repeats until the issue clears or power is reset.
🛑 Important Rule Before You Decode Anything
Before touching anything:
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Turn the thermostat off
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Observe the code during a normal call for heat
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Write it down
Resetting power too early erases valuable information.
🔥 The Most Common Goodman Furnace Diagnostic Codes (Explained)
Below are the codes I see most often — and what they actually mean in real homes.
🟡 Code 1: Normal Operation
What it means:
The furnace is operating correctly.
Jake’s take:
If you’re getting heat and see Code 1, stop worrying. This is confirmation, not a problem.
🌬️ Code 2: Pressure Switch Stuck Closed
What the furnace is saying:
“I expected the pressure switch to be open before startup, and it wasn’t.”
Common causes:
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Blocked vent or intake pipe
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Moisture in the pressure switch hose
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Pressure switch failure
Homeowner check:
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Look for snow, debris, or nests in intake/exhaust pipes
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Check for visible water in clear hoses
If it returns, this is pro territory.
💨 Code 3: Pressure Switch Open (Inducer Problem)
What it means:
The furnace can’t prove proper draft.
Common causes:
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Blocked venting
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Condensate drain issues
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Inducer motor problems
Why this matters:
This code protects against flue gas spillage and unsafe combustion.
Jake’s rule:
Pressure switch codes are never “ignore and run anyway” situations.
🔥 Code 4: Open High Limit Switch (Overheating)
This one matters a lot.
What the furnace is saying:
“I’m too hot. I shut down to protect myself.”
Most common causes:
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Dirty air filter
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Blocked supply or return vents
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Dirty blower wheel
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Improper airflow
Homeowner action:
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Replace the filter immediately
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Make sure vents are open and unblocked
If this happens repeatedly, call a pro — overheating damages heat exchangers.
⚡ Code 5: Flame Sensed Without Call for Heat
What it means:
The furnace senses flame when it shouldn’t.
Possible causes:
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Stuck gas valve
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Dirty flame sensor
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Control board issues
Jake’s take:
This is not a DIY diagnosis. Shut the system down and call for service.
🔥 Code 6 or 7: Ignition Failure
What the furnace is saying:
“I tried to light, but flame wasn’t proven.”
Common causes:
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Dirty flame sensor
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Gas supply issues
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Ignitor problems
Safe homeowner step:
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Power cycle once
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If it returns, stop
Repeated ignition failures stress components and flood burners with gas.
🌡️ Code 9: Reversed Polarity or Grounding Issue
What it means:
The furnace senses incorrect electrical polarity.
Common causes:
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Outlet wiring issues
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Improper grounding
This is an electrical safety issue — not a furnace adjustment.
🔁 Why Codes Appear Even When the Furnace “Still Works”
This is where homeowners get confused.
Many Goodman furnaces:
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Log faults
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Recover automatically
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Keep running
That doesn’t mean the issue is gone.
Repeated codes = developing problems, not false alarms.
🧠 How Diagnostic Codes Protect the Heat Exchanger
Most safety codes exist to prevent:
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Overheating
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Flame rollout
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Incomplete combustion
Ignoring codes often leads to:
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Short cycling
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Metal fatigue
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Premature exchanger failure
The control board is trying to save expensive parts.
🛠️ Safe DIY Checks vs “Call a Pro” Lines
✅ Homeowner-safe checks:
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Replace air filter
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Clear vent/intake pipes
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Observe and record codes
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Power reset once
🚫 Call a professional if:
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Codes repeat
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Gas-related codes appear
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Pressure switch issues persist
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CO detectors activate
Guessing saves nothing.
📋 Jake’s Diagnostic Code Survival Checklist
✔ Write the code down
✔ Check airflow first
✔ Never bypass safety switches
✔ Don’t reset repeatedly
✔ Respect repeated faults
Follow that list and you’ll never make a bad call.
🔗 Verified External Resources
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Goodman Gas Furnace Overview:
https://www.goodmanmfg.com/products/gas-furnaces -
Furnace Safety & Controls (Energy.gov):
https://www.energy.gov/energysaver/furnaces-and-boilers -
Carbon Monoxide Safety (CDC):
https://www.cdc.gov/carbon-monoxide/about/index.html -
Indoor Air Quality & Combustion Safety (EPA):
https://www.epa.gov/indoor-air-quality-iaq
🔚 Jake’s Final Word
Diagnostic codes aren’t warnings of doom — they’re early conversations.
Listen to them, write them down, and respond appropriately. Do that, and your Goodman furnace will:
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Protect itself
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Protect your home
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Avoid unnecessary breakdowns
Ignore them, and small problems quietly become big ones.
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In the next topic we will know more about: Vent & Flue Care: Why Your Furnace’s ‘Breathing’ System Needs Yearly Inspection
