Display shows A04, boiler does not start, low pressure symbol may be lit

Safety: Allow the system to cool before topping up. Do not exceed the maximum pressure indicated in the Wolf manual.

• Read the pressure gauge: Check the current system pressure. If it is below the minimum (often 0.8–1.0 bar), the boiler will lock out with A04.
• Top up via filling loop:
  • Locate the filling loop (flexible hose or built-in filling device).
  • Open the valves slowly while watching the gauge.
  • Increase pressure to the recommended cold value (usually around 1.2 bar).
  • Close the valves firmly and remove the hose if detachable.
• Bleed radiators: If you hear gurgling, bleed radiators to remove air, then re-top up pressure.
• Check for visible leaks: Inspect radiators, valves, and pipework for drips. Also check the boiler’s condensate trap area and safety valve discharge pipe outside.
• If pressure drops repeatedly: Contact a heating engineer to test the expansion vessel pre-charge and inspect for hidden leaks or a faulty pressure sensor.