Troubleshooting Common Furnace Issues: When to DIY and When to Call a Pro

Tony’s Field Guide to Fixing (and Respecting) Your Furnace

There’s a good reason Tony gets calls every winter from frantic homeowners saying, “My furnace just died!” Most of the time, it’s not dead — just misunderstood.

“Nine times out of ten, the furnace isn’t broken. It’s dirty, neglected, or tripped out by something simple. The other one time? That’s when experience — and a license — matter.”

If you’ve got a Goodman 80% AFUE 120,000 BTU Two-Stage Gas Furnace (Model GR9T801205DN) and it’s acting up, don’t panic. This guide will walk you through Tony’s step-by-step process to figure out what’s wrong, what you can fix yourself, and when it’s time to call in a pro.

---

🏠 Not Every Furnace Problem Means a Panic Call

Tony likes to say that furnaces have personalities. Sometimes they’re just grumpy — not broken.

He recalls one customer who called him out at 9 p.m. for “no heat.” The issue?

“The furnace switch at the top of the stairs was off. They thought it was a light switch. Cost them $150 for me to flip it.”

The truth: most furnace problems fall into one of three categories:

1. Simple DIY fix (filter, switch, thermostat).

2. Moderate issue (sensor cleaning, minor reset).

3. Professional repair (gas, electrical, or control system fault).

Knowing which category yours is in can save hundreds — and keep you safe.

---

🔧 Step 1: Start with the Basics — Power, Thermostat & Filter

Before diving into panels and sensors, Tony always starts simple.

🔌 Check Power Supply

• Make sure the furnace switch (often mistaken for a light switch) is ON.

• Check the breaker box — if tripped, reset it once. If it trips again immediately, stop and call a pro.

🌡️ Thermostat Check

• Set to HEAT mode.

• Temperature at least 3°F above current room temp.

• Ensure FAN is set to AUTO (not ON).

🧼 Filter Check

A dirty air filter can restrict airflow so badly the furnace overheats and shuts down for safety.

Tony’s rule:

“If your filter looks gray instead of white, your furnace is suffocating.”

Replace filters every 2–3 months — more often if you have pets or live in a dusty area.

✅ DIY: Safe and simple.
🚫 Pro Needed: Only if breaker keeps tripping.

🔗 Resource: Energy.gov – HVAC Maintenance Basics

---

🔥 Step 2: No Heat (Blower Runs, but No Flame)

This is one of the most common calls Tony gets. The fan blows, but you’re getting nothing but cold air.

Common Causes

• Gas valve closed

• Dirty flame sensor

• Igniter failure

• Safety switch lockout

Tony’s Quick Test

1. Check the gas valve: The handle should be parallel to the pipe (open).

2. Listen during startup:

   • Click → whoosh = ignition working

   • Click → silence = ignition failed

3. Clean the flame sensor:

   • Shut power off.

   • Remove the small metal rod near the burner.

   • Gently polish it with fine emery cloth.

   • Reinstall and power back up.

If you hear multiple click attempts followed by shutdown, the igniter or gas valve likely needs replacement.

“Cleaning sensors is fine. Messing with gas valves isn’t. Know your line.”

✅ DIY: Flame sensor cleaning, reset power
🚫 Pro Needed: Igniter or gas valve repair

🔗 Read: HVAC.com – Furnace Not Igniting?

---

🌬️ Step 3: Weak or Uneven Airflow

Tony calls this one the “lukewarm complaint.” The house heats, but not evenly — or airflow feels weak.

Likely Causes

• Clogged filter

• Blocked or disconnected ductwork

• Blower speed misconfiguration

• Failing motor capacitor

DIY Fix

1. Replace filter (yes, again).

2. Open all supply and return vents.

3. Check for loose duct joints in the basement.

4. Vacuum register grilles.

If airflow is still weak, the ECM blower motor might not be ramping properly.

Tony explains:

“The Goodman’s 9-speed motor adjusts airflow automatically. If it’s sluggish or noisy, the capacitor or control board could be the culprit — that’s pro territory.”

✅ DIY: Filter, vents, duct inspection
🚫 Pro Needed: Motor or capacitor repair

🔗 Learn: Smarter House – Duct System Efficiency

---

🔁 Step 4: Short Cycling — Furnace Turns On and Off Repeatedly

Short cycling means the furnace starts, runs briefly (2–5 minutes), then shuts off before reaching temperature.

Causes

• Dirty filter (again!)

• Blocked vents

• Oversized furnace

• Faulty thermostat

• High-limit switch tripping

Tony’s Troubleshooting

1. Replace filter and clear vents.

2. Ensure thermostat isn’t near a heat source (like sunlight or a vent).

3. Open all interior doors for airflow.

If the problem persists, your limit switch may be tripping due to overheating — often from restricted airflow or poor duct balance.

“Short cycling kills furnaces fast. Every on-off adds wear that shortens its life.”

✅ DIY: Filter, airflow, thermostat
🚫 Pro Needed: Limit switch, control board, or sizing evaluation

---

🔊 Step 5: Strange Noises — Bangs, Whines, or Rattles

Tony can diagnose a furnace just by listening.

Common Sounds

Tony’s tip:

“A healthy furnace should hum, not holler. Don’t ignore new noises — they only get louder.”

✅ DIY: Tighten panels
🚫 Pro Needed: Any mechanical sound

---

🌡️ Step 6: Furnace Blowing Cold Air

Your furnace starts but never warms the house — frustrating, but fixable.

Causes

• Thermostat set to ON instead of AUTO

• Dirty filter or blocked return

• Ignition failure after startup

• Faulty flame sensor

Tony’s Test

1. Check fan mode — should be AUTO.

2. Replace filter and restart.

3. Watch ignition through sight glass:

   • No flame = igniter issue.

   • Flame starts then dies = dirty sensor.

If you’ve cleaned the sensor and still get cold air, call for a diagnostic on the gas valve or control board.

✅ DIY: Thermostat, filter, reset
🚫 Pro Needed: Gas or ignition components

🔗 Guide: Consumer Reports – Furnace Troubleshooting Basics

---

⚠️ Step 7: Unusual Smells or Odors

Tony always tells homeowners: trust your nose.

Common Odors & Causes

“If it smells off, shut it off. That’s my golden rule.”

🚨 Safety Tip: Always have a carbon monoxide detector near your furnace and test it monthly.

🔗 Resource: NFPA – Heating Safety Guidelines

---

🧯 Step 8: Furnace Keeps Tripping the Breaker

This one gets Tony’s full attention. It’s a warning sign of an electrical overload.

Possible Causes

• Overloaded shared circuit

• Shorted blower motor

• Damaged wiring or transformer

Tony’s Steps

1. Unplug any devices on the same circuit.

2. Reset breaker once.

3. If it trips again, do not reset again — call a pro.

“A breaker trips because it’s protecting you from fire. Don’t ‘fix’ that with a bigger fuse.”

✅ DIY: First reset only
🚫 Pro Needed: Anything recurring

---

🧠 Tony’s DIY vs. Pro Decision Guide

Here’s Tony’s simple chart for when to pick up a wrench — and when to pick up the phone.

Tony sums it up:

“If there’s gas, electricity, or high heat involved — step away. That’s not DIY territory.”

---

🧰 Tony’s Troubleshooting Toolkit

A few simple tools can help you safely check the basics:

• Flashlight

• ¼” nut driver (panel access)

• Fine emery cloth (for sensor cleaning)

• Multimeter (check voltage)

• Handheld CO detector

• Fresh filters

“You don’t need fancy gadgets — just clean parts, clear airflow, and a little patience.”

---

💡 How to Prevent Furnace Problems Before They Start

Tony swears by prevention. His own Goodman furnace has run flawlessly for years thanks to these habits:

1. Change filters every 90 days

2. Schedule a fall tune-up before first use

3. Vacuum intake and return vents seasonally

4. Keep area around furnace clear of clutter

5. Install surge protector for blower and board protection

6. Register your Goodman warranty (10-year parts, lifetime heat exchanger)

---

🧾 Tony’s “First Five-Minute Fix” Checklist

If your furnace stops working suddenly:

1. Check power switch and breaker

2. Verify thermostat is on HEAT & AUTO

3. Replace or inspect filter

4. Ensure gas valve is open

5. Reset furnace (cut power 30 seconds, then restore)

“If it still doesn’t kick on, that’s your cue to call in help. Don’t let curiosity turn into a safety issue.”

---

💬 Tony’s Takeaway: “Fix What You Can — Respect What You Can’t”

“I love when homeowners want to understand their furnaces. The more you know, the less you spend. But when it comes to gas, ignition, or electrical, the smartest move isn’t doing it yourself — it’s knowing when not to.”

Goodman furnaces are some of the most dependable systems around. But even the best need attention, inspection, and a little teamwork between homeowner and technician.

Tony’s final word:

“You keep it clean, I’ll keep it safe. Together, that furnace will last decades.”

✅ Final Word

Troubleshooting your Goodman 120,000 BTU Two-Stage Furnace isn’t about becoming a repair tech — it’s about awareness. With Tony’s step-by-step approach, you’ll know when to handle small fixes confidently and when to hand it over to the pros.

“A furnace is like a truck,” Tony says. “You can check the oil and clean the air filter yourself — but if it starts smoking, you don’t rebuild the engine in your driveway.”

In the next topic we will know more about: Gas Furnace Safety 101: Proper Venting, Carbon Monoxide & Combustion Tips