E9 error, unit stops, and may show water leakage or abnormal defrost behavior.

Safety: Turn off power before working near water and electrical components.

• Check for visible leaks: Inspect around the indoor unit for water dripping. Place a container or towels to protect walls and floors.
• Clean drain line: Locate the condensate drain outlet near the outdoor unit or wall. Use a wet/dry vacuum on the outlet to suck out blockages. Alternatively, blow compressed air gently from the indoor side if accessible.
• Inspect float switch: Some Vaillant units have a float switch in the drain pan. Open the front panel and lower cover to access the pan. Clean any slime or debris and ensure the float moves freely.
• Check slope: Verify that the drain hose slopes continuously downward with no upward loops.
• Test: After cleaning, pour a small amount of water into the drain pan to confirm it flows freely out of the drain.
• If E9 persists: The drain pan level sensor or float switch may be faulty and should be replaced.