Error 4 flashes, 5 second gap, 3 long flashes

Safety precautions:

1) If you smell gas, leave the property immediately and call the gas emergency service. Do not operate switches or the boiler. 2) If the boiler has been hot or you see steam, allow it to cool down before touching pipework. 3) Do not attempt to bypass or defeat safety devices (sensors, thermostats, limiter). 4) If in doubt, switch off the boiler at the electrical isolator and shut the gas supply to the appliance until a qualified engineer attends.

Initial checks a homeowner can safely do:

1) Note the exact fault LED sequence and any other symbols or messages on the boiler. Take a photo for the engineer. 2) Try a single reset following the manual: press the reset or turn the control knob to the reset position for 3 seconds. Wait and observe. If the boiler restarts and runs normally for a period, monitor it closely; a repeat trip indicates an underlying problem. Do not repeatedly reset the boiler. 3) Check the system pressure on the pressure gauge (usually about 1.0–1.5 bar cold). If pressure is very low, re-pressurise the system following the boiler manual instructions and then try a single reset. 4) Check that all radiator valves and the boiler flow/return isolation valves are open and that the room thermostat/programmer is calling for heat if appropriate. 5) Listen at the boiler when calling for heat: can you hear the pump running? If the pump is noisy, vibrating, or apparently not running while the boiler is trying to fire, that points to circulation issues. 6) Bleed radiators if air is suspected to ensure proper circulation.

Specific diagnostic and next-step actions (when fault persists):

1) If the boiler trips immediately or shortly after firing and you have already tried a single reset and basic circulation checks, switch the boiler off and call a Gas Safe engineer. 2) Do not open the boiler casing or attempt to test internal sensors, wiring, or replace components yourself. The engineer will measure flow and return temperatures, test the max safety thermostat and temperature sensors, verify pump speed and operation, check the diverter valve and flow paths for blockages, and inspect the heat exchanger for scale or restriction. 3) If the engineer finds a seized/sticking pump, blocked heat exchanger or heavy scale, they will clear or replace the component as required. If a sensor or the max safety thermostat itself is faulty, the engineer will test and replace the sensor/thermostat and check wiring/connectors and the PCB as needed. 4) If pressure or filling issues are the cause, the engineer will check system integrity and the expansion vessel. 5) After repair, the engineer will verify correct operation, run the boiler to normal temperatures, and ensure the safety thermostat does not re-trip.

When to call a professional and what to prepare for the engineer:

1) Call a Gas Safe registered engineer if the fault persists after a single reset and basic checks, if you can’t restore circulation, if radiators remain cold, or if you detect abnormal smells, noises, leaking, or signs of overheating. 2) Provide the engineer with the boiler model, serial number, the exact fault flash sequence (4 quick, 5 second gap, 3 long), when it started, any recent changes or servicing history, and the results of any checks you carried out (pressure, pump sounds, reset attempts). 3) Leave the boiler isolated and safe if there are any safety concerns until the engineer arrives.

Final note: This error is a safety shutdown and should not be ignored. Basic checks and one controlled reset are reasonable for a homeowner, but diagnosis and repair of overheating, circulation faults, or replacement of safety thermostats/sensors must be done by a qualified Gas Safe engineer for safety and warranty reasons.