Boiler displays E04 and stops, temperature on display rises rapidly before shutdown.

Safety: Turn off power and allow the boiler to cool fully before touching any pipework. Surfaces may be extremely hot.

• Check system pressure: Verify that the pressure gauge on the Defro boiler shows within the recommended range (typically 1.0–1.5 bar when cold). If pressure is very low, top up via the filling loop according to the manual, then bleed radiators.
• Verify circulation: Ensure all isolation valves on the flow and return pipes beneath the Defro boiler are fully open. Check that thermostatic radiator valves are not all closed.
• Inspect pump operation: With the front cover removed and power on, call for heat and listen/feel the Defro circulation pump. If it is silent or excessively hot to touch, it may be seized. Some pumps have a front screw that can be opened (with power off) to manually free the shaft using a screwdriver.
• Clean system filter: If a Defro system filter or magnetic filter is installed on the return line, isolate it, remove the cap, and clean out sludge. Restore flow and check for leaks.
• Reset overheat stat: Some Defro models have a manual reset overheat thermostat on the top of the heat exchanger. Once cooled, press the reset button if present. If it trips repeatedly, call a technician to check for scale or blockage in the primary heat exchanger.