🎤 Hey There: No Heat? Don’t Panic – Mark’s Diagnostic Deep Dive
Alright, gather ’round the burner box, folks. Mark here.
There’s nothing like waking up at 6 a.m. on a cold January morning, ready to grab some coffee—and BAM, the house is ice-cold. No hum from the furnace, no airflow, just dead silence and an LED flashing like Morse code from furnace purgatory.
Sound familiar? If you’re running one of these 5-ton R-32 AC and gas furnace combo systems, the cooling might be high-tech, but your furnace still runs on gas, flame sensors, control boards, and a few pieces that haven’t evolved much since the 90s. That means real-world, hands-on troubleshooting still matters.
This blog is your full guide to understanding why your furnace might fail, how to fix it, and when to call in backup before you blow a fuse—or worse, your temper.
Let’s keep that heat on and the call to the service tech off.
🔥 Ignition Failures: When the Flame Just Won’t Start
The most common (and most annoying) problem I see on gas furnaces? It won’t light. You’ll hear the draft inducer kick on, maybe a click, and then… nothing.
Causes usually fall into these categories:
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Dirty or failed flame sensor
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Cracked hot surface ignitor
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Gas valve not opening
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No signal from the board
What to do:
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Remove and clean the flame sensor with fine-grit emery cloth
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Check resistance on the hot surface ignitor (should be 40–90 ohms)
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Use a multimeter to see if 24V is getting to the gas valve
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Confirm polarity—yep, reversed polarity can kill ignition
And remember, R-32 systems may share a circuit with the condenser—make sure the furnace is getting full voltage independently. If you’re not sure how to check polarity and voltage safely, check out this basic furnace ignition guide from HVAC School.
🌡️ High Limit Switch Tripping: Overheating Without a Flame
The high limit switch is your furnace’s safety net. If it gets too hot inside the heat exchanger, the switch opens and kills the burners. But that doesn’t mean the furnace is broken—it means something else failed first.
Common causes:
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Clogged air filter
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Closed or undersized supply/return ducts
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Failed blower motor or capacitor
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Oversized furnace in undersized ductwork
Want to verify? Use a clamp meter on the blower motor. Check amps and compare to spec. Also test the temperature rise across the heat exchanger. The data plate will tell you the allowed range (usually 40–70°F). Go over that, and you’re cooking the plenum—and not in a good way.
Need a walkthrough on airflow and duct sizing? The experts at Energy Vanguard explain it with real numbers, not just guesses.
⚠️ Control Board LED Codes: Decoding the Blinking Lights
Furnace not starting but flashing a light at you like it’s trying to communicate from the other side? That’s the control board diagnostic LED. Every brand has a different pattern, but they all mean something.
Here’s a sample from Goodman’s boards:
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1 blink: Ignition failure
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2 blinks: Pressure switch stuck closed
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3 blinks: Pressure switch stuck open
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4 blinks: Open high-limit
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Continuous blink: Normal operation
Grab your model number and head to Goodman’s tech support site to pull the board’s specific error code guide.
Don’t ignore those codes—they’ll save you hours of guesswork. Just remember: the code points to the symptom, not always the root cause.
🌬️ Blower Motor Not Running: No Air, No Heat
Let’s say the burners are firing, but there’s no airflow. That means your blower isn’t spinning, and that could point to:
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Failed motor (especially ECM motors with age)
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Blown run capacitor
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Bad relay on the board
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Loose wiring harness
To diagnose:
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Try spinning the blower wheel by hand—if it’s stiff or stuck, bearings are toast
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Test capacitor microfarads—if it’s 15μF and reads 3μF? Replace it
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Check board outputs with a voltmeter. If you're not getting 120V to the motor, blame the board
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Listen for buzzing or clicking—classic capacitor or relay failure symptoms
Want a primer on blower motors? HVAC Know It All covers ECM vs PSC diagnostics like a champ.
🧱 Pressure Switch Failures: Stuck, Broken, or Just Miswired
Your pressure switch is a tiny diaphragm that confirms the draft inducer is working and the heat exchanger is clear. If it doesn’t close (or open), the furnace locks out before ignition.
Look for:
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Blocked flue pipe
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Clogged drain on condensing units
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Loose tubing
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Water trapped in pressure switch line
What I do:
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Pull the hose, blow it clear
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Bypass the pressure switch briefly (never leave it that way) to confirm function
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Clean the barb connectors and make sure tubing fits tight
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Use a manometer to test inches of water column across the switch
Be cautious—some techs immediately replace the switch when the real issue is water in the vent. Learn the difference and save yourself a callback.
🧼 Flame Rollout and Soot: Dirty Burners, Dirty Results
A rollout switch trips when flames come out the front of the burner instead of going into the heat exchanger. That’s serious. Don’t just reset and move on.
Root causes include:
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Clogged burner orifices
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Cracked heat exchanger
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Negative pressure in the utility room
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Excessive gas pressure or improper mix
If your furnace smells like soot or leaves black marks? Shut it down and inspect.
You can read more about rollout hazards and detection in ESFI’s safety materials.
🧯 Limit Switch Lockouts: Why Does It Keep Failing?
If your system constantly hits the reset lockout, it may not be the component’s fault. It could be cycling on limit or flame signal dropout. Try this checklist:
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Check flame rectification signal (should be 1.5–4.5 microamps DC)
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Check for voltage drops across neutral
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Make sure you're not sharing a neutral between furnace and lighting
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Confirm duct static pressure is within spec
A simple manometer + combustion analyzer can save hours of trial and error here. I once replaced a board on a Goodman that only needed a grounded wire fixed. Embarrassing.
🧢 Wrap Up: Stay Warm, Stay Smart – Mark’s Final Words
Here’s the truth, friends: furnaces are dumb machines—but they follow logic. If your 5-ton combo system’s furnace isn’t firing right, don’t just poke around and hope. Follow the sequence of operation. Read the codes. Grab your meter and let the readings tell the story.
This isn’t rocket science, but it is life safety. So don’t take shortcuts. Gas and flame demand respect.
Keep a journal of your calls. Write down what went wrong and how you fixed it. That way, next time you’re staring at a flashing LED at 10pm on a holiday weekend, you’ll know where to start.
And hey—if you haven’t bought your 5-ton R-32 system yet, get one that’s matched, tested, and supported by folks who actually know HVAC. I recommend starting with The Furnace Outlet’s lineup. These aren’t mystery boxes from the bargain bin. They’re built for performance and backed by real documentation—and that matters when you’re 10 degrees from frostbite.
Want to take all the right steps to make sure your 5-ton R-32 system runs perfectly? Visit my guide: Commissioned to Perform.
Until next time,
– Mark Callahan 🧢🛠️
"Still checking for gas leaks, still fixing your cousin’s ‘install,’ still not impressed by that flex duct you used as a return plenum."
