Error 3 Green Flashes

Safety first:

1) If you smell gas, leave the property immediately, avoid operating electrical switches or using mobile phones in the building, and call the emergency gas number from a safe location. 2) If the boiler is showing lockout red lights or there is any sign of flame failure that you cannot resolve, do not keep trying to force it to run — get a qualified engineer.

Initial checks a homeowner can do:

1) Note the exact display and flashing pattern and take a photo. Check if the fault is steady or intermittent. 2) Check central heating pressure on the gauge - aim for roughly 1.0–1.5 bar when cold. If pressure is low, top up using the filling loop following the boiler manual. 3) Check that other gas appliances are working to confirm gas supply is present. 4) Check external flue and air vents are not blocked by debris, leaves or snow. 5) Check condensate pipe (if fitted) for freezing in cold weather and thaw with warm (not boiling) water if frozen. 6) Check room thermostat and timer settings and any wireless controls or smart thermostats to make sure they are calling for heat correctly.

Reset procedure (one-time attempt):

1) Locate the boiler reset button or selector (refer to the boiler label/manual). 2) Press and hold the reset for about 5–10 seconds and release. 3) Allow the boiler to attempt a restart and watch for the display to clear and the burner to light. 4) If the boiler restarts and runs normally, monitor for recurrence. Do not repeatedly reset more than once or twice if it keeps returning.

Simple diagnostics and checks you can do safely:

1) If the fault returns, record when it happens (cold start, after several hours, after pump run-on) and any accompanying symptoms (loss of hot water, noisy pump, pressure drop). 2) Do a visual check of accessible wiring and connectors to external controls and the thermostat (only if the outer casing is closed and power is off at the isolator) to ensure plugs are seated and no obvious damage exists. Do not open or tamper with internal PCB covers or gas components. 3) Bleed one or two radiators to clear trapped air and re-check pressure. 4) If you suspect a frozen condensate pipe, thaw it and then reset the boiler.

When to call a professional (Gas Safe engineer):

1) If the green-flash error repeats or the boiler locks out (red flashes) after reset. 2) If you notice persistent loss of flame, ignition attempts failing, persistent low or dropping pressure, strange smells, unusual noises, or any internal error codes that do not clear. 3) If you are not comfortable performing the checks above or if any work would involve opening the boiler, gas fittings, the PCB or fan/ignition components. A qualified engineer will carry out safe, diagnostic tests (electrical continuity, sensor checks, gas pressure and valve operation, PCB diagnostics) and replace faulty components if required.

Important cautions:

1) Do not attempt internal electrical or gas repairs yourself. 2) Do not repeatedly reset the boiler if the fault returns — this can mask a worsening issue and may stress components. 3) Always use a Gas Safe registered engineer for gas-side and PCB/electrical faults. Provide the engineer with photos and a clear record of when the fault occurs to speed diagnosis.

If the fault was truly a one-off and cleared after reset, continue to monitor. If it returns, contact a Gas Safe engineer and share the flashing pattern, any stored error codes and the steps you’ve already taken.