Boiler shows A04, no heating or hot water, pump may run briefly then stop

Safety: Allow the boiler to cool before working on the system. Do not over-pressurise above the red zone on the gauge.

Steps:

• Check the system pressure gauge on the IVT boiler. It should typically be around 1.0–1.5 bar when cold (check your manual).
• If below 1.0 bar, locate the filling loop (flexible hose with two valves). Open both valves slowly and watch the gauge rise.
• When the pressure reaches the recommended value, close both valves fully and remove the filling loop if it is detachable.
• Reset the boiler and observe operation. If pressure drops again, inspect visible pipework and radiators for leaks.
• Bleed radiators to remove trapped air, then re-top up pressure as needed.

If A04 persists despite correct pressure, a technician should:

• Test and replace the system pressure sensor / switch if faulty.
• Check for hidden leaks and expansion vessel failure.