How to Tell if Flame Sensor is Bad: A Comprehensive Guide

The flame sensor is a critical component in a furnace or boiler system, responsible for detecting the presence of a flame and ensuring safe operation. When the flame sensor is not functioning correctly, it can lead to various issues, including the furnace shutting down unexpectedly or failing to ignite at all. Knowing how to properly diagnose and test a flame sensor can save you time, money, and potential safety hazards. In this comprehensive guide, we’ll explore the key indicators and techniques to determine if your flame sensor is bad.

Measuring Microamps (uA)

The flame sensor should produce a small electrical current, typically between 4-6 microamps (uA), when exposed to a flame. This current can be measured using a digital multimeter set to the microamp (uA) setting. If the current is significantly lower, it may indicate that the flame sensor is dirty, damaged, or failing.

To perform this test, follow these steps:

1. Turn off the power to the furnace or boiler.
2. Locate the flame sensor, which is usually a metal rod or probe near the burner assembly.
3. Set your multimeter to the microamp (uA) setting.
4. Disconnect the flame sensor from the control board or wiring harness.
5. Expose the flame sensor to a flame, such as a lit match or lighter.
6. Measure the current output of the flame sensor. It should be between 4-6 uA.
7. If the current is significantly lower, the flame sensor may need to be cleaned or replaced.

Checking Continuity

The flame sensor should have continuity, meaning there is a continuous electrical path through the sensor. To test for continuity, use a digital multimeter set to the continuity or resistance (Ω) setting.

1. Turn off the power to the furnace or boiler.
2. Locate the flame sensor and disconnect it from the control board or wiring harness.
3. Set your multimeter to the continuity or resistance (Ω) setting.
4. Touch the multimeter probes to the two terminals of the flame sensor.
5. The multimeter should indicate continuity (a beep or a low resistance reading).
6. If there is no continuity, the flame sensor is likely broken and will need to be replaced.

Visual Inspection

Visually inspecting the flame sensor can reveal signs of damage or contamination that may be affecting its performance. Look for the following:

• Cracks or damage to the porcelain insulation
• Buildup of soot, debris, or corrosion on the metal rod or probe
• Discoloration or signs of overheating

If you notice any of these issues, the flame sensor may need to be cleaned or replaced.

Analyzing Flame Color

The color of the flame can also provide clues about the condition of the flame sensor. A healthy, properly functioning furnace or boiler should have a blue flame. If the flame appears yellow or orange, it may indicate that the gas is not burning completely, which could be a sign of a problem with the flame sensor or other components.

Checking for Flue Line Corrosion

Corrosion in the flue line can be a sign that the flame sensor is shutting down the heating system due to a leak or other issue. Inspect the flue line for any signs of rust, pitting, or deterioration, as this may indicate a problem that the flame sensor is detecting and responding to.

Monitoring Energy Bills

Unexpectedly high energy bills can be a symptom of a malfunctioning flame sensor. If the flame sensor is not properly detecting the presence of a flame, the furnace or boiler may be running inefficiently, leading to increased fuel consumption and higher utility costs.

Bench Testing the Flame Sensor

To thoroughly test the flame sensor, you can set up a bench test using a burner (such as a Bunsen burner) and a power source. This allows you to directly expose the flame sensor to a flame and measure its current output, as well as check for continuity and visual signs of damage.

1. Set up a bench test with a burner and power source.
2. Disconnect the flame sensor from the furnace or boiler.
3. Insert the flame sensor into the flame and measure the current output using a digital multimeter set to the microamp (uA) setting.
4. The current should be between 4-6 uA. If it is significantly lower, the flame sensor may need to be cleaned or replaced.
5. Check the continuity of the flame sensor using a digital multimeter set to the continuity or resistance (Ω) setting.
6. Visually inspect the flame sensor for any signs of damage or contamination.

By performing these comprehensive tests, you can accurately diagnose whether your flame sensor is functioning properly or in need of repair or replacement.

Cleaning and Maintaining the Flame Sensor

If the flame sensor is not producing the expected current or showing signs of contamination, you can try cleaning it before replacing it. Follow these steps:

1. Turn off the power to the furnace or boiler.
2. Disconnect the flame sensor from the control board or wiring harness.
3. Use a small amount of fine-grit sandpaper or emery cloth to gently remove any soot, debris, or corrosion from the metal rod or probe.
4. Wipe the sensor clean with a soft, lint-free cloth.
5. Reattach the flame sensor and turn the power back on.
6. Test the sensor again to ensure it is producing the correct current output.

Regular maintenance and cleaning of the flame sensor can help extend its lifespan and prevent premature failure.

Conclusion

Diagnosing and testing a flame sensor is a critical step in maintaining the safe and efficient operation of your furnace or boiler. By understanding the key indicators, technical specifications, and testing procedures, you can quickly identify and address any issues with the flame sensor, ensuring your heating system continues to function reliably.

References

• How can I bench test a Flame Sensor (Flame Rectification)?
• Checking micro amps on flame sensor
• 6 Signs the Flame Sensor in Your Furnace Has Gone Bad
• 3 Symptoms of a Bad Flame Sensor & What to Do
• Bad Flame Sensor on Furnace