Boiler displays E04 and will not run, overheat light may be on

Safety: Allow the boiler to cool fully before attempting any checks. Do not bypass safety thermostats.

• Check system pressure: Ensure the pressure gauge reads around 1.0–1.5 bar when cold. If low, top up via the filling loop following the Main user instructions and bleed radiators.
• Bleed air from radiators: Use a radiator key to release trapped air, starting with the highest radiators. Re-pressurise as needed.
• Confirm valves are open: Check the flow and return isolation valves under the Main boiler are fully open.
• Listen for pump operation: On a heat demand, place a hand on the pump body (if accessible) to feel for vibration. No movement suggests a seized or failed pump.
• Engineer tasks:
  • Reset or replace the manual reset overheat thermostat on the Main primary heat exchanger if fitted.
  • Check for sludge or scale blockage in the primary heat exchanger and system pipework; powerflush if necessary.
  • Test the CH flow and return NTC sensors for correct resistance vs temperature.
  • Verify correct pump speed and head; replace the Main pump assembly if defective.