Furnace Flame Sensor Explained: Symptoms, Cleaning, and When to Replace It
What Every Winnipeg Homeowner Should Know About This Critical Furnace Safety Part
When your furnace turns on but shuts off after a few seconds, many Winnipeg homeowners assume the problem is serious — and sometimes expensive. In reality, one of the most common causes is a dirty or faulty furnace flame sensor.
The flame sensor is a small but critical safety device designed to confirm that your furnace is burning fuel safely. When it fails or becomes dirty, your furnace will shut down immediately to prevent dangerous gas buildup.
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What Is a Furnace Flame Sensor?
A furnace flame sensor is a thin metal rod, usually mounted near the burner assembly, with a single wire connected to it. Its only job is to detect whether a flame is present once the furnace ignites.
If the sensor does not detect a flame within a few seconds, the furnace immediately shuts off the gas supply as a safety measure.
This protects your home from:
- Gas leaks
- Carbon monoxide buildup
- Fire hazards
How a Furnace Flame Sensor Works (Flame Rectification)
The flame sensor works using a principle called flame rectification.
Here’s how it happens:
- The sensor sends a tiny electrical current through the flame
- The flame acts as a conductor
- The current returns through the burner and furnace chassis
- The control board confirms safe combustion and keeps gas flowing
If the sensor cannot detect this current, the furnace assumes there is no flame — even if one exists — and shuts down.
Common Signs of a Bad or Dirty Flame Sensor
A failing flame sensor usually causes very specific symptoms:
Furnace Short Cycling
The furnace ignites, runs for a few seconds, then shuts off repeatedly.
No Heat
The furnace may attempt to start but never stays on long enough to heat your home.
Yellow or Weak Flames
This may indicate incomplete combustion or sensor-related issues that should be inspected immediately.
Frequent Error Codes
Many modern furnaces show diagnostic LED codes related to flame detection failures.
Why Flame Sensors Fail (Especially in Winnipeg Homes)
The most common issue is dirt and soot buildup.
Other causes include:
- Carbon deposits from combustion
- Dust in older furnaces
- High humidity or condensation
- Cracked or worn sensors
- Loose wiring or control board issues
Winnipeg’s long heating season means flame sensors work hard — making regular inspection important.
Can You Clean a Furnace Flame Sensor Yourself? (DIY Guide)
⚠ Important: Always turn off power to the furnace before attempting any work.
Basic Cleaning Steps
- Turn off furnace power at the breaker
- Locate the flame sensor (thin metal rod near burners)
- Carefully remove the mounting screw
- Gently clean the rod using fine-grit sandpaper or a soft cloth
- Avoid touching the rod with bare hands
- Reinstall and restore power
In many cases, simple cleaning restores normal furnace operation.
Testing a Flame Sensor (Advanced)
Professional technicians may test the sensor using a multimeter:
- Set meter to DC microamps
- A healthy sensor usually reads 1–2 microamps
- Readings below this range often indicate a failing sensor
If cleaning does not restore proper readings, replacement is recommended.
When Should a Flame Sensor Be Replaced?
Replace the flame sensor if:
- Cleaning does not fix short cycling
- The sensor is cracked or damaged
- Microamp readings are consistently low
- The furnace still shuts down unexpectedly
Flame sensors are relatively inexpensive, but correct diagnosis is critical — other issues like airflow or gas pressure can cause similar symptoms.
For accurate diagnosis, professional inspection is always the safest option.
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Furnace Flame Sensor Problems? We Can Help
If your furnace won’t stay on, shuts off randomly, or is showing flame-related error codes, don’t wait — especially during Winnipeg’s cold winters.
LIDOMA Home Services provides expert furnace diagnostics, cleaning, and repair across Winnipeg.
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