Troubleshooting Furnace Won’t Stay Lit: Not a Flame Sensor Issue

by

When your furnace won’t stay lit, it can be a frustrating and concerning problem. While a dirty flame sensor is a common culprit, there are several other potential causes that need to be addressed. This comprehensive guide will walk you through the various reasons why your furnace may not be staying lit, as well as provide detailed troubleshooting steps to help you get your heating system back up and running.

Understanding the Furnace Ignition Sequence

To effectively troubleshoot a furnace that won’t stay lit, it’s important to understand the basic ignition sequence of a furnace. The typical sequence is as follows:

  1. Thermostat Call for Heat: The thermostat senses a drop in temperature and sends a signal to the furnace to begin the heating cycle.
  2. Blower Motor Activation: The blower motor starts, circulating air through the furnace and ductwork.
  3. Gas Valve Opening: The gas valve opens, allowing gas to flow to the burners.
  4. Igniter Activation: The igniter, either a hot surface igniter or a spark igniter, is activated to ignite the gas.
  5. Flame Sensor Verification: The flame sensor detects the presence of a flame and sends a signal to the control board to keep the gas valve open.
  6. Burner Ignition: The burners ignite, and the furnace begins heating the air.
  7. Thermostat Satisfied: Once the desired temperature is reached, the thermostat signals the furnace to shut off.

If the furnace is not staying lit, the issue may occur at any of these steps in the sequence.

Potential Causes of a Furnace Not Staying Lit

furnace wont stay lit not flame sensor

  1. Dirty or Malfunctioning Flame Sensor:
  2. The flame sensor is responsible for detecting the presence of a flame and signaling the control board to keep the gas valve open.
  3. If the flame sensor is dirty, coated with soot, or malfunctioning, it may not be able to properly detect the flame, causing the furnace to shut off.
  4. Cleaning the flame sensor with fine grit sandpaper or steel wool can often resolve this issue.

  5. If the flame sensor is damaged, it will need to be replaced.

  6. Faulty Thermocouple:

  7. The thermocouple is a safety device that monitors the pilot light or main burner flame and shuts off the gas supply if the flame is extinguished.
  8. A malfunctioning thermocouple can cause the furnace to shut off, even if the flame is present.

  9. Checking the thermocouple for proper operation and replacing it if necessary can help resolve this problem.

  10. Tripped Pressure Switch:

  11. The pressure switch is a safety device that monitors the airflow in the furnace and shuts off the gas supply if it detects improper venting or airflow issues.
  12. If the pressure switch is tripped, it can cause the furnace to shut off, even if the flame is present.

  13. Checking for any blockages or obstructions in the air intake or exhaust vent, as well as ensuring the pressure switch is functioning correctly, can help address this issue.

  14. Insufficient Airflow:

  15. Inadequate airflow for combustion can prevent the furnace from staying lit.
  16. This can be caused by blocked or dirty air filters, obstructions in the ductwork, or issues with the blower motor.

  17. Replacing the air filter and inspecting the ductwork for any blockages can help improve airflow and allow the furnace to stay lit.

  18. Gas Supply Issues:

  19. Problems with the gas supply, such as low gas pressure or a malfunctioning gas valve, can prevent the furnace from staying lit.

  20. Checking the gas line for any leaks or blockages, as well as ensuring the gas valve is functioning correctly, can help resolve this issue.

  21. Electrical Issues:

  22. Problems with the furnace’s electrical components, such as a faulty control board, wiring issues, or a malfunctioning igniter, can cause the furnace to not stay lit.

  23. Checking the electrical connections, testing the control board, and replacing any faulty components can help address these electrical problems.

  24. Thermostat Malfunction:

  25. A malfunctioning thermostat can send incorrect signals to the furnace, causing it to not stay lit.

  26. Checking the thermostat for proper operation, including its ability to accurately sense temperature and communicate with the furnace, can help identify and resolve this issue.

  27. Pilot Light Issues:

  28. If the furnace has a pilot light, a problem with the pilot light, such as it being extinguished or not burning properly, can prevent the furnace from staying lit.
  29. Relighting the pilot light and ensuring it is burning steadily can help resolve this problem.

To effectively troubleshoot a furnace that won’t stay lit, it’s essential to methodically work through each of these potential causes, starting with the most common issues and progressing to more complex problems. By following a systematic approach and utilizing the appropriate tools and safety precautions, you can often identify and resolve the underlying issue, restoring your furnace to proper operation.

Troubleshooting Steps

  1. Inspect the Flame Sensor:
  2. Locate the flame sensor, which is typically a metal rod near the burners.
  3. Use fine grit sandpaper or steel wool to gently clean the flame sensor, removing any soot or debris buildup.

  4. If the flame sensor is severely damaged or corroded, it will need to be replaced.

  5. Check the Thermocouple:

  6. Locate the thermocouple, which is typically a metal rod near the pilot light or main burners.
  7. Visually inspect the thermocouple for any signs of damage or corrosion.
  8. Use a multimeter to test the thermocouple’s resistance and ensure it is within the manufacturer’s specified range.

  9. If the thermocouple is faulty, it will need to be replaced.

  10. Inspect the Pressure Switch:

  11. Locate the pressure switch, which is typically a small device connected to the furnace’s air intake or exhaust vent.
  12. Check for any blockages or obstructions in the air intake or exhaust vent that could be causing the pressure switch to trip.
  13. Use a multimeter to test the pressure switch’s continuity and ensure it is functioning correctly.

  14. If the pressure switch is malfunctioning, it will need to be replaced.

  15. Assess Airflow:

  16. Check the air filter and replace it if it is dirty or clogged.
  17. Inspect the ductwork for any blockages or obstructions that could be restricting airflow.

  18. Ensure the blower motor is functioning correctly and providing adequate airflow.

  19. Verify Gas Supply:

  20. Check the gas line for any leaks or blockages that could be restricting the gas supply.
  21. Ensure the gas valve is fully open and functioning correctly.

  22. Use a manometer to measure the gas pressure and verify it is within the manufacturer’s specified range.

  23. Inspect Electrical Components:

  24. Check all electrical connections to the furnace, ensuring they are tight and free of corrosion.
  25. Test the control board, igniter, and other electrical components using a multimeter to ensure they are functioning correctly.

  26. Replace any faulty electrical components as needed.

  27. Troubleshoot the Thermostat:

  28. Ensure the thermostat is set to the correct mode (heat) and the desired temperature is set higher than the current room temperature.
  29. Check the thermostat’s wiring and connections to the furnace.
  30. Test the thermostat’s ability to accurately sense temperature and communicate with the furnace.

  31. Replace the thermostat if it is malfunctioning.

  32. Address Pilot Light Issues:

  33. If the furnace has a pilot light, ensure it is lit and burning steadily.
  34. Check the pilot light assembly for any obstructions or issues that could be preventing the pilot light from staying lit.
  35. Adjust the pilot light flame as necessary, following the manufacturer’s instructions.

By systematically working through these troubleshooting steps, you can often identify and resolve the underlying issue causing your furnace to not stay lit, without it being a problem with the flame sensor.

Remember to always prioritize safety when working on your furnace, and consider contacting a professional HVAC technician if you are uncomfortable or unsure about any of the troubleshooting steps.

Reference: