{"product_id":"oem-lennox-flame-sensor-21l99","title":"OEM Lennox Flame Sensor — 21L99","description":"\u003cp\u003eThe genuine OEM Lennox \u003cstrong\u003e21L99\u003c\/strong\u003e flame sensor is a critical safety component in your gas furnace. It uses flame rectification to confirm that the burners have actually ignited, signaling the control board to keep the gas valve open. When a valid flame is detected, your furnace runs normally; when it is not, the board shuts off the gas to prevent a dangerous unburned-fuel buildup.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eCommon failure symptoms:\u003c\/strong\u003e A failing or dirty \u003cstrong\u003e21L99\u003c\/strong\u003e sensor is one of the most frequent causes of short-cycling. You may notice the furnace lighting for a few seconds and then shutting down, repeated ignition attempts, no heat despite the blower running, or a control board flashing a flame-sense or lockout fault code. Oxidation and carbon buildup on the sensor rod reduce the microamp signal until the board no longer trusts it.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eWhy OEM matters:\u003c\/strong\u003e Lennox designs the 21L99 to specific rod length, alloy, and mounting geometry so it produces the exact flame-rectification current the Lennox control board expects. Generic sensors often sit at the wrong depth in the flame or produce a weak signal, leading to nuisance lockouts. Choosing the OEM part ensures proper fit, a stable microamp reading, and rated safe operation across compatible Lennox, Armstrong, and Ducane furnaces.\u003c\/p\u003e","brand":"Lennox","offers":[{"title":"Default Title","offer_id":63081112699251,"sku":"LNX-21L99","price":19.95,"currency_code":"USD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0984\/1190\/5395\/files\/nhvac-flame-sensor-straight-LNX-21L99.jpg?v=1783761018","url":"https:\/\/nationalhvacparts.com\/products\/oem-lennox-flame-sensor-21l99","provider":"National HVAC Parts","version":"1.0","type":"link"}