Display shows F4, unit does not start, fan may run briefly

Safety: Isolate electrical power and allow the unit to cool. Exhaust components can be very hot.

• Locate flue gas sensor: On most Viessmann water heaters, the flue gas NTC sensor is clipped to the flue outlet or integrated into the flue collar. Disconnect the sensor plug and inspect for corrosion or loose pins.
• Check sensor resistance: With power off, measure the resistance of the flue gas sensor using a multimeter and compare to the Viessmann temperature/resistance chart. Replace the sensor if readings are out of range or open/shorted.
• Inspect flue for blockage: Check the entire flue run for obstructions, sagging, or water accumulation. Ensure termination is clear of snow, leaves, or bird nests.
• Check heat exchanger condition: If the unit is older or on hard water, excessive scale can cause overheating. Inspect the primary heat exchanger surfaces for heavy deposits. Descaling should be performed by a professional using Viessmann-approved chemicals.
• Reset and test: After correcting issues, restore power, clear the fault, and run the unit while monitoring flue temperature via the service menu if available.