Error 4 flashes green

Safety precautions:

1) If you can smell gas, leave the property immediately and call the national gas emergency number—do not try to investigate. 2) Before doing any checks that involve the boiler’s internals, isolate electrical power at the boiler fused spur or consumer unit and switch off the gas supply if you plan to open the casing. 3) If you are not competent with electrics or working inside gas appliances, stop and call a Gas Safe registered engineer.

Initial homeowner checks (no internal access required):

1) Note the fault: confirm the boiler is showing 4 green flashes consistently. Write down any other lights or display messages. 2) Try a simple reset: follow the boiler’s reset procedure (press and hold the reset button or move selector to R for the required seconds). Wait for the boiler to attempt restart and observe if the fault returns. 3) Power cycle: switch the boiler off at the isolator or spur for 30–60 seconds, then switch back on and observe. 4) Check external controls: ensure programmer/timer, room thermostat and any wireless controllers are powered, correctly set, and working. Replace dead batteries, confirm schedules and that the room stat calls for heat. Where possible, try temporarily bypassing a separate thermostat (only if you know how to do this safely) to check if the communication fault is with an external control. 5) Inspect visible wiring and terminations that are accessible without opening the boiler (e.g., thermostat wires at external terminals, programmer connections). Look for loose screws, damaged insulation or obvious faults. Do not open the boiler to inspect internal wiring unless competent and isolated from mains and gas.

Specific diagnostic and next steps (if fault persists):

1) If the fault reappears after the reset and basic checks, avoid repeated resets. Repeated lockouts can mask an intermittent fault and may cause further issues. 2) If you have multiple controllers (room stat, external sensor, smart control), disconnect non-essential external controls one at a time (power down first) to identify if one external device is causing the communication failure. 3) If the visible external wiring and controls are fine and the fault continues, it is likely an internal wiring or PCB/combustion control unit fault. At this stage call a Gas Safe registered engineer. Provide the engineer with the exact fault (4 green flashes), what you have already tried (resets, power cycle, external checks) and whether the fault is constant or intermittent.

What a professional will do (information for you, not a DIY instruction):

- Isolate and safely open the boiler, then inspect internal harnesses, connectors and earths for loose, corroded or damaged connections. - Measure voltages and check continuity between the User Interaction Controller/display and the main combustion control board. - Check for software/firmware issues, blown or stressed components, and signs of water ingress, overheating or vibration damage. - Replace faulty wiring, replace the User Interaction Controller or combustion control unit / PCB where required and re-test. - Run a full functional test and ensure the boiler returns to normal operation without recurring communication errors.

Final notes and when to call a professional:

- If the fault clears after a single reset and does not return, monitor the boiler but report it if it reappears. - If the fault is recurring, permanent, or any part of the troubleshooting requires opening the boiler, call a Gas Safe registered engineer. - Do not attempt to replace PCBs, gas valves or perform internal electrical repairs yourself unless you are a qualified gas/heating engineer. Safety-critical components and gas appliance work must be carried out by a competent professional.