Water heater shuts down with F3, burner stops shortly after ignition

Safety: Turn off power and gas before opening the combustion chamber. Do not operate the unit with covers removed.

• Check burner for contamination: Remove the burner assembly following the Viessmann service instructions. Inspect burner ports for dust, lint, or corrosion. Clean with a soft brush and vacuum; do not enlarge ports.
• Inspect gas valve connections: Ensure the gas valve inlet and outlet unions are tight and there are no kinks in the gas line. Do not adjust internal gas valve settings unless you are certified.
• Check air intake and flue: Verify that the concentric flue/air intake system is not blocked by debris, snow, or nests. Ensure all flue joints are properly sealed and not recirculating exhaust into the intake.
• Reset and monitor: Restore power and gas, reset the fault, and run the heater. If the flame lights and then drops out repeatedly, the gas valve modulation or control board may be defective and should be tested by a Viessmann-trained technician.