Furnace indicates condensate overflow or blocked drain; unit shuts down to protect.

Safety first: Turn off power before working on condensate components. Water may spill; protect surrounding area.

Steps:

• Locate the CTC condensate trap and drain tubing at the base of the secondary heat exchanger or collector box.
• Disconnect the tubing and trap. Flush them thoroughly with warm water and a small amount of mild detergent to remove slime and debris.
• Use a wet/dry vacuum at the drain termination (outside or at floor drain) to pull any blockage from the line.
• If equipped, inspect the CTC condensate float switch in the drain pan. Ensure it moves freely and is not stuck in the up (tripped) position.
• Reassemble, ensuring all joints are tight and sloped properly toward the drain.
• Restore power and run the furnace, checking for proper condensate flow.