Lennox 21L99 Flame Sensor

Professional-grade Lennox 21L99 Flame Sensor is a critical OEM replacement part designed to maintain pilot and burner safety through precise flame rectification. As a veteran-preferred component, this sensor is engineered to monitor the furnace flame and provide the necessary DC microamp signal to the control board, ensuring the gas valve remains energized only when a safe ignition is confirmed. It serves as a direct OEM replacement for the Lennox 21L9901, offering a seamless fit for many Lennox, Armstrong, and Ducane gas-fired furnaces.

The 21L99 features an L-shaped rod construction with high-temperature porcelain insulation to prevent grounding issues. For technicians troubleshooting ignition failures, this sensor typically generates a signal between 2 and 6 microamps in a healthy system. If you are seeing furnace short cycling or ignition lockout codes, checking this flame rod for carbon buildup or a cracked ceramic insulator is a standard diagnostic step. This Lennox furnace part is built for the demanding environments of residential and light commercial heating equipment, providing the reliable flame sensing required by modern integrated furnace controls.

Key Benefits:

• Direct Compatibility: Exact OEM 21L99 replacement for 21L9901, ensuring proper alignment in the burner flame path.
• High-Temperature Durability: Constructed with a robust stainless steel rod and high-grade porcelain insulation to withstand intense thermal stress.
• System Safety: Vital for preventing unburned gas accumulation by providing rapid flame-out detection.
• Broad Application: Compatible with a wide range of Lennox HVAC models, including LGA and LGC series units.

Common Questions:

What is the standard microamp reading for the 21L99?
Most Lennox boards look for a signal above 1.0 µA to maintain operation, though a clean sensor should ideally produce 2.0 to 5.0 µA when properly enveloped in the flame.

Can I use this to replace the 21L9901?
Yes, the 21L99 is the official Lennox replacement part for the older 21L9901 part number.

Why does my furnace light and then shut off after 5 seconds?
This is the classic symptom of a failed flame sensor. If the control board does not receive the rectified DC signal within the ignition window, it will close the gas valve as a safety precaution.

Disclaimer: While we strive for 100% accuracy, product data and specifications are subject to change. Please contact our team to confirm compatibility or technical details before purchasing.