Error 4 green flashes

Safety precautions:

1) If you smell gas, do not touch the boiler. Open windows, leave the property and call your gas emergency number immediately. 2) Turn off electrical power to the boiler at the isolator switch before attempting any physical inspection. 3) Do not remove sealed covers or attempt PCB replacement unless you are a qualified Gas Safe engineer—there are live electrical parts and gas safety implications. 4) Avoid repeated resets; excessive resetting can mask a developing fault and may stress components.

Initial checks a homeowner can do (no tools or minimal tools):

1) Confirm the code: verify the boiler is showing exactly 4 green flashes and note any other lights or messages. 2) Note boiler behaviour: is heating and hot water working normally, intermittent, or completely unavailable? 3) Power reset: try a simple reset using the boiler reset button (hold for 5–10 seconds) or switch the boiler off at the isolator/power switch, wait 30–60 seconds, then turn it back on. Many temporary comms errors clear after a reset. 4) Check mains power and fuse spur: ensure the boiler is getting stable power and that the fuse or RCD feeding it hasn’t tripped. 5) Check external controls: ensure room thermostat, programmer/timer, and any smart controls are powered and calling for heat; a faulty external control or loose connection to it can look like a comms fault.

Diagnostic and further checks (basic, safe steps):

1) Watch and log behaviour: if the fault clears after a reset, monitor frequency and record date/time and what other events happened (power cut, thermostat change). Intermittent faults that return are important information for an engineer. 2) Inspect for obvious external signs of damage: with power off, look for water ingress at the boiler casing, signs of corrosion, scorch marks, or rodent damage to external wiring to the control or programmer. Do not open the PCB compartment. 3) Check connectors you can access safely: if your boiler manual shows an external connector between a separate control panel and the main unit and that connector is accessible without removing sealed panels, ensure it is fully seated. Do not probe or unplug internal cables without isolating power and without competence. 4) Check the action log or diagnostics display if your model provides one—this may record more detailed messages or a repeating pattern that helps the engineer.

When to call a professional and what to tell them:

1) Call a Gas Safe registered engineer if the 4 green flashes persist after a reset, if the fault returns repeatedly, or if any other error lights/codes appear alongside it. Also call an engineer if you are not comfortable performing the basic checks above. 2) Provide the engineer with the boiler model, the exact error (4 green flashes), when it started, frequency, whether the boiler recovers after reset, the results of your initial checks (power cycle, external components checked), and any related symptoms (loss of heat/hot water, audible clicking, water around the unit). 3) The engineer will perform safe electrical and continuity tests, check connector integrity, inspect the PCB, PSU/PU and control panel, and may replace or re-seat faulty connectors or defective modules. They will also check for underlying causes such as water damage, blown components, or intermittent supply voltage problems.

Final notes and cautions:

1) Temporary 4-green-flash events that self-clear are usually low risk, but repeated occurrences should not be ignored. 2) Do not attempt internal PCB repair, component swaps, or gas system work yourself. These tasks require a qualified Gas Safe engineer for safety and warranty reasons. 3) If the boiler locks out on other codes (red flashes) or you notice smells, leaks, loud unusual noises or loss of heat/hot water, isolate the appliance and call a professional immediately.