Boiler indicates gas valve or gas flow control fault; ignition may fail or flame may be unstable.

Critical safety warning: Gas valve diagnosis and replacement must only be carried out by a Gas Safe / certified gas engineer. Do not attempt to open or adjust the gas valve yourself.

User‑level checks:

• Ensure the external gas isolation valve to the boiler is fully open.
• Check that other gas appliances in the property work normally (if not, there may be a supply issue).
• Power‑cycle the boiler using the main switch and attempt a reset as per the Bosch manual.

Technician actions (for reference only): A professional will check gas valve coil resistance, verify correct voltage from the PCB during ignition, and measure gas pressures and combustion values. If the valve or PCB is faulty, they will replace and recommission the boiler.