Lennox 34M77 Furnace Pressure Switch OEM Replacement - 65289100

The Lennox 34M77 is an OEM replacement pressure switch designed for use in Lennox gas furnaces and air handlers. Also referenced by OEM part number 65289100, this safety switch monitors the negative pressure created by the draft inducer blower motor to confirm proper venting before allowing the ignition sequence to proceed.

Pressure switches are a critical safety component in modern induced-draft and condensing furnaces. If the inducer fails to generate adequate airflow — due to a blocked flue, failed motor, or cracked pressure port — the switch remains open, shutting down the heating cycle and preventing the buildup of harmful combustion gases.

Key Features:

• OEM Lennox replacement pressure switch — direct fit for compatible Lennox units
• Cross-reference OEM part number: 65289100
• Monitors draft inducer pressure to ensure safe furnace operation
• Prevents ignition if proper venting pressure is not detected
• Helps maintain safe, code-compliant furnace operation
• Manufactured to OEM specifications for reliable performance

When replacing a faulty pressure switch, always inspect the rubber hose, pressure port, and drain lines for blockages or cracks before installation, as these are common root causes of pressure switch failures. Refer to your furnace service manual for wiring details and set-point specifications.