Water heater locks out with F1 on display; burner does not start.

Safety first: Turn off power to the Viessmann water heater at the breaker and close the gas shutoff valve before opening the unit. Allow the unit to cool fully.

• Inspect the ionization electrode: Remove the front cover. Locate the ionization (flame) electrode near the burner. Check for soot, scale, or corrosion. Clean the electrode gently with fine emery cloth or a Scotch-Brite pad. Ensure the ceramic insulator is not cracked.
• Check electrode position: Verify the tip of the ionization electrode is correctly positioned in the burner flame path per the service manual (typically a few millimeters above the burner surface). Adjust the mounting bracket if bent.
• Inspect wiring: Trace the ionization electrode cable back to the control board. Ensure the spade connector is tight, no burn marks are present, and no damaged insulation. Reseat the connector.
• Verify burner grounding: Confirm the burner assembly has a solid ground connection to the chassis. Tighten ground screws and clean any rust or paint under the ground lug.
• Reset and test: Restore power and gas, then reset the fault using the control panel. Run a hot water draw and observe ignition. If F1 returns, the flame sensing circuit on the main PCB may be defective and should be tested or replaced by a qualified technician.