Boiler shows E04 and will not start, overheat or safety thermostat has tripped.

Safety first: Turn off power and allow the boiler to cool completely. Do not repeatedly reset an overheating boiler without finding the cause.

• Check system pressure: On sealed systems, verify that the pressure gauge shows the correct cold pressure (typically 1.0–1.5 bar, check boiler label). If low, top up via the filling loop and bleed radiators.
• Bleed air from boiler: Use the manual air vent on the boiler or pump (if fitted) to release trapped air. Have a cloth ready for water drips.
• Verify pump operation: With power restored, call for heat and listen/feel for the circulation pump running. If the pump is silent or very hot to touch, it may be seized or failed and should be replaced.
• Reset overheat thermostat: Locate the manual reset overheat thermostat on the Weishaupt boiler (often under the front panel). Press the reset button firmly. If it trips again, stop using the boiler and investigate further.

When to call a professional: If the overheat thermostat repeatedly trips, if the pump is not running, or if there are signs of scale or sludge blocking the heat exchanger, contact a qualified heating engineer.