OEM Lennox Limit Switch — 20J99

The genuine OEM Lennox 20J99 limit switch is a critical safety control that monitors temperature inside your furnace heat exchanger and plenum. When temperatures climb beyond the safe operating threshold, this high-limit switch interrupts the burner circuit to prevent overheating, while also signaling the blower to run and clear excess heat. It is a frontline defense protecting both your Lennox furnace and your home.

Common failure symptoms: A faulty 20J99 can cause the furnace to short cycle — igniting and shutting off repeatedly — or fail to fire at all. You may also experience the blower running constantly, no heat on call, or a system that locks out on a limit fault. A switch that trips prematurely or sticks open leaves your furnace unable to complete a heating cycle.

Why OEM matters: Because this is a safety-rated component, the exact trip temperature and electrical rating must match your Lennox furnace design. The factory-original 20J99 is calibrated to Lennox specifications, ensuring correct protection setpoints and reliable operation. Using a non-OEM substitute risks improper overheat protection and possible nuisance faults. Insist on genuine Lennox for safety and a precise fit.