Error 3 flashes 5 seconds off

Safety first:

- If you smell gas (rotten egg odour or gas smell), do not touch the boiler, do not operate electrical switches, open windows, turn off the gas at the meter if it is safe to do so, leave the building and call the national gas emergency number (in the UK 0800 111 999).

- If you suspect carbon monoxide (e.g. soot, yellow/orange flame, illness), turn off the appliance, ventilate, leave the property and get emergency help.

- If no gas smell, isolate the boiler electrically (turn off the mains switch) before touching any external controls. Do not open the boiler casing or attempt internal repairs unless you are Gas Safe registered.

Initial homeowner checks (safe, non-invasive):

1. Note the exact fault pattern and any static cause code. Write down 3 flashes with 5s gap and the time/date and any other displayed numbers.

2. Try a controlled reset: follow the manual for your model (usually hold the reset button or turn the control knob to the reset position for ~3 seconds). After resetting, wait and call for heat or hot water to see if the fault reappears. Do not keep repeatedly resetting a boiler that locks out more than once.

3. Check other gas appliances in the property (hob, gas fire) to confirm the property gas supply is present. If none work, check the gas meter isolation valve and call your gas supplier.

4. Check mains electrical supply to the boiler (fuse, external isolation switch) to ensure it has power. Record if the boiler shows any other lights or messages.

5. If your boiler has a spanner/return button or information menu, press it to view the cause code (three-digit cause number). Record this cause code — it is useful to the engineer.

Specific diagnostic notes (information to gather, do not attempt internal fixes):

- After resetting once, observe the boiler’s ignition sequence (if visible or by sound). Does it attempt to light then lock out? Any unusual noises? Does the ignition electrode spark? These observations help the engineer identify whether the issue is electrical, ignition, ionisation sensor or the gas valve itself.

- If the fault is intermittent (works sometimes, then faults), this can indicate a poor electrical connection to the gas valve, failing gas valve, or a failing PCB. If the fault appears every time, it more likely points to a failed valve, failed valve driver on the PCB, or a failed safety/leak test.

- If the boiler shows additional codes like FA/FB/FA 364/FB 365 in the info menu, these specifically point to gas valve leak-test failures and must be handled by a Gas Safe engineer.

What NOT to do:

- Do not dismantle or attempt to repair or replace the gas valve, PCB, ignition electrode, or wiring if you are not qualified. Those components involve gas tightness, electrical safety and combustion tuning.

- Do not repeatedly reset the boiler as a long-term workaround — repeated lockouts are dangerous and indicate an unresolved fault.

When to call a professional and what to tell them:

- Call a Gas Safe registered engineer whenever the gas valve error persists after a single controlled reset, appears repeatedly, or you see additional cause codes in the info menu.

- Provide the engineer with: the exact flash sequence (3 flashes 5s off), any cause code from the information menu, when the fault started, whether the fault is permanent or intermittent, whether other gas appliances are working, and whether you smell gas.

Likely professional actions (for your information only):

- The engineer will perform safe gas-tightness checks, electrical and continuity checks on the gas valve and its connectors, check the PCB outputs and ignition/ionisation circuitry, and run manufacturer diagnostics and leak tests. They will only replace the gas valve or PCB if required and will perform any necessary pressure and combustion checks.

Summary guidance:

- A single, careful reset is reasonable to try. If the error returns or if you detect a gas smell, isolate the boiler and call emergency services or your Gas Safe engineer. Any further diagnosis or repair of the gas valve, wiring or internal controls must be carried out by a qualified Gas Safe registered professional.