How to Troubleshoot Your Furnace Before Calling a Pro

Key Takeaways

• Check the thermostat, filter, and power switch before calling for service
• A dirty flame sensor causes 75% of "furnace won't stay lit" calls
• Never attempt gas line or heat exchanger repairs yourself
• Know when DIY troubleshooting ends and professional help begins

In This Article

1. Before You Start: Safety First
2. 8 Common Furnace Issues You Can Fix
3. When to Call a Professional
4. FAQ

Your furnace stopped working and the house is getting cold. Before you pick up the phone and schedule a furnace repair visit — which typically costs $150–$350 in the GTA — try these furnace troubleshooting steps. About 30% of service calls turn out to be issues homeowners can diagnose and fix themselves in under 15 minutes, saving both time and money.

This guide walks through the eight most common furnace problems, what causes them, and which ones are safe to handle on your own versus which ones need a licensed technician.

Before You Start: Safety First

Gas furnaces involve combustion, electrical components, and natural gas — all of which can be dangerous if mishandled. Follow these ground rules:

• If you smell gas (a rotten egg odour), leave the house immediately and call Enbridge Gas at 1-866-763-5427. Do not flip any switches or use your phone inside the home.
• If your CO detector is alarming, evacuate and call 911. Carbon monoxide is colourless and odourless — trust the detector.
• Never remove the furnace's front panel while the unit is running. Turn off the power switch (usually a standard light switch on or near the unit) before inspecting internal components.

8 Common Furnace Issues You Can Fix Yourself

1. Thermostat Set Incorrectly

Symptom: Furnace doesn't turn on at all.
Check: Is the thermostat set to "Heat" (not "Cool" or "Off")? Is the temperature set above the current room temperature? On battery-powered models, replace the batteries — low power causes erratic behaviour. If you have a programmable thermostat, check that the schedule hasn't overridden your manual setting.

Fix: Adjust settings. If the display is blank, replace batteries or check the breaker for a hardwired unit.

2. Dirty or Clogged Air Filter

Symptom: Furnace runs but produces weak airflow, or it short-cycles (turns on and off every few minutes).
Check: Pull the filter out and hold it up to a light. If you can't see through it, it's restricting airflow. A severely clogged filter triggers the high-limit safety switch, which shuts the furnace down to prevent overheating.

Fix: Replace the filter. This single step resolves more furnace complaints than any other. For help picking the right one, see our guide to furnace filter types.

3. Furnace Power Switch Turned Off

Symptom: Furnace is completely unresponsive — no fan, no ignition, no display.
Check: Look for a standard light switch on or near the furnace. It's easy to flip accidentally, especially if your furnace is in a utility closet or laundry room. Also check the circuit breaker panel for a tripped breaker labelled "Furnace" or "HVAC."

Fix: Flip the switch on. Reset the breaker. If the breaker trips again immediately, stop — you have an electrical issue that needs a technician.

4. Condensate Drain Blockage

Symptom: High-efficiency furnace (90%+ AFUE) shuts down and won't restart. You may see water pooling near the base.
Check: High-efficiency condensing furnaces produce water as a byproduct. This water drains through a small plastic tube. If the tube clogs — often from mineral buildup or algae — a safety switch shuts the furnace off.

Fix: Disconnect the drain tube and flush it with warm water and white vinegar. Ensure the drain outlet (usually a floor drain or utility sink) is clear. If the condensate pump failed, that's a technician repair ($200–$350 installed).

5. Pilot Light or Ignition Failure

Symptom: Furnace tries to start — you hear the blower — but no heat comes out.
Check: On older furnaces with a standing pilot light, look through the inspection window. The pilot should be a steady blue flame. If it's out, follow the manufacturer's relighting instructions printed on the unit. On modern furnaces with electronic ignition, you may see a blinking LED error code on the control board.

Fix: Relight the pilot following instructions. For electronic ignition issues, note the LED blink code (e.g., 3 flashes = "pressure switch stuck open") and report it when you call for service. This gives the technician a head start.

6. Blower Runs Continuously

Symptom: The fan never stops, even when the furnace isn't heating.
Check: Your thermostat's fan setting. If it's set to "ON" instead of "AUTO," the blower runs 24/7 regardless of whether the furnace is producing heat.

Fix: Switch the fan setting to "AUTO." If the blower still runs continuously on AUTO, the fan relay on the control board may be stuck — that requires a technician.

7. Closed or Blocked Registers

Symptom: Some rooms are warm, others are cold. The furnace seems to be working fine.
Check: Open every supply register in your home. Check that furniture, drapes, and rugs aren't blocking return air grilles. Closing more than 20% of registers creates back-pressure that can damage the blower motor and ductwork.

Fix: Open all registers. If airflow is still uneven, the issue may be undersized or leaky ductwork — a problem our technicians diagnose during a maintenance plan visit.

8. Dirty Flame Sensor

Symptom: Furnace ignites, runs for 3–10 seconds, then shuts off. Repeats the cycle.
Check: The flame sensor is a small metal rod that detects whether the burners actually lit. When it's coated with oxidation, it can't sense the flame, and the control board shuts off the gas valve as a safety precaution. This is the #1 reason furnaces short-cycle.

Fix: If you're comfortable removing the furnace panel: turn off power, locate the flame sensor (thin metal rod near the burners), gently clean it with fine sandpaper or steel wool, and reinstall. If you're not comfortable with this, a technician can do it in about 10 minutes for $100–$150.

When to Call a Professional

Some furnace problems are beyond safe DIY territory. Call a licensed HVAC technician immediately if:

• You smell gas or suspect a gas leak
• The heat exchanger shows visible cracks or rust
• The furnace produces a burning or electrical smell
• Error codes indicate a gas valve, pressure switch, or control board failure
• The circuit breaker trips repeatedly when the furnace runs
• You've tried all the steps above and the problem persists

If your furnace is more than 15 years old and repairs are becoming frequent, it may be more cost-effective to replace your furnace rather than keep repairing it. Our article on 7 warning signs it's time for a new furnace can help you decide.

Frequently Asked Questions

Don't Get Stuck in the Cold

Furnace troubleshooting starts with the simple stuff: thermostat, filter, power switch. These three checks resolve about a third of all service calls. For everything else, a licensed technician has the tools and training to diagnose and fix the problem safely.

Need help now? Call H&C at 1-855-539-4328 for same-day furnace repair across the GTA, or request a free quote online.

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