Boiler shows A4 and will not start; may occur after power interruption or service work

Safety: Allow the boiler to cool. Do not overfill the system; excessive pressure can cause leaks.

• 1. Check pressure gauge: Look at the Buderus boiler's pressure gauge. If below the recommended cold pressure (often 1.0–1.5 bar), the system needs topping up.
• 2. Refill system: Locate the filling loop (flexible hose or built-in filling device). Open the valves slowly and watch the gauge rise. Close the valves when the correct pressure is reached.
• 3. Reset boiler: After refilling, reset the boiler according to the user instructions and check if A4 clears.
• 4. Inspect for leaks: If pressure drops again, inspect radiators, valves, and visible pipework for leaks. Also check the boiler underside and safety valve discharge pipe.
• 5. Pressure sensor check: If the gauge shows correct pressure but the boiler still reports A4, the electronic pressure sensor or pressure switch may be faulty. Inspect its wiring and connector; replacement may be required.
• 6. Expansion vessel: A failed expansion vessel can cause frequent pressure loss via the safety valve. This should be checked and recharged or replaced by a technician.