Furnace shows E4 and shuts down, often after running for a short time

Safety first: Turn off power and allow the furnace to cool fully before touching any internal components.

• Check the system pressure on the gauge; if very low, top up to the recommended pressure (typically around 1.5 bar when cold) using the filling loop, then bleed radiators.
• Ensure all radiator and isolation valves on the flow and return are open.
• Listen for the circulation pump when the unit starts. If it is silent or excessively noisy, it may be seized or air-locked. Carefully vent the pump via the bleed screw (have a cloth ready for small water leaks).
• Inspect the primary heat exchanger for signs of scale or sludge build-up (discoloured water, noisy boiling sounds). Heavy fouling requires professional cleaning or replacement.
• Locate the manual reset overheat thermostat (if fitted) on the heat exchanger and press the reset button once the unit is cool.
• If the E4 returns frequently, arrange for a technician to check flow rates, pump performance, and heat exchanger condition.