No ignition, control shows ignition fault, burner does not light at all

Safety: High voltage is present at the ignition transformer. Disconnect power before inspection.

• Visual inspection: Remove the front cover and locate the ignition transformer module and cable. Look for burn marks, melted plastic, or loose connectors.
• Check electrode gap and position: Remove the ignition electrode and verify the gap and position according to the Viessmann service manual (typically 3–4 mm). Clean the tips with fine abrasive and reinstall.
• Check cable continuity: Flex the ignition cable gently; if the insulation is cracked or stiff, replace the cable. Ensure both ends are firmly seated.
• Test for spark (technician recommended): A professional can safely test for spark output from the transformer. If no spark is produced, the ignition transformer or control board must be replaced.
• Do not bypass safety: Never attempt to manually light the burner or bypass ignition controls. If F4 persists after basic checks, schedule service.