Furnace does not start; display shows low water pressure or lockout related to system pressure.

Safety first: Turn off power before working on any water components. Do not over‑pressurize the system.

• Check pressure gauge: On Sime hydronic furnaces, the gauge should typically read around 1.0–1.5 bar when cold (consult your model label). If below the minimum, the low‑pressure switch will prevent operation.
• Top up via filling loop: Locate the filling loop (flexible braided hose with two valves) beneath the furnace. Open the valves slowly while watching the gauge. Close both valves once the correct pressure is reached. Wipe any drips.
• Bleed radiators: If pressure drops again, air may be trapped. Bleed radiators starting from the highest floor, then re‑top the system to the correct pressure.
• Inspect for leaks: Look for damp patches under the furnace, at radiator valves, and along visible pipework. Persistent pressure loss indicates a leak that must be repaired.
• Check pressure sensor: If pressure is correct but the A04 error persists, the pressure switch or sensor on the Sime hydraulic group may be faulty and should be tested and replaced by a technician.