Boiler indicates a low system pressure condition; heating may not start.

Safety: Allow the system to cool before topping up. Do not over-pressurise the boiler.

Steps:

• Check pressure gauge: On the front of the Bosch boiler, note the pressure. If below about 1.0 bar when cold, topping up is required.
• Top up via filling loop: Locate the filling loop (flexible hose or built-in valves). Open the valves slowly until the pressure rises to 1.0–1.5 bar, then close both valves fully.
• Bleed radiators: Bleed any air from radiators, then recheck pressure and top up again if needed.
• Check for leaks: Inspect visible pipework, radiators, and the boiler underside for signs of water leakage.

If pressure continues to drop, there may be a hidden leak or faulty expansion vessel; a heating engineer should investigate.