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Safety precautions:

- If you smell gas or a carbon monoxide alarm sounds, evacuate the property immediately, call the gas emergency service (in the UK call 0800 111 999) and do not re-enter until the scene is declared safe. Do not attempt further checks.

- If no immediate danger, switch the boiler off at the control and at the electrical isolator before doing any close inspection. Do not remove the boiler casing or attempt repairs to gas or electrical components yourself unless you are qualified.

Initial checks a homeowner can do:

1. Note the exact fault display pattern and any additional codes or action-log entries. Write them down for the engineer.

2. Perform a single reset: press the boiler reset button or move the selector to R and hold for about 5 seconds (follow your model’s reset method). Wait and observe whether the boiler attempts to restart and whether the error returns. Do not keep repeatedly resetting if the fault reappears.

3. Check the external flue terminal outside the house for obvious blockages such as bird nests, leaves, ice or debris and clear any safe, external obstructions you can reach without climbing or unsafe access. A blocked flue can affect fan/flue readings.

4. Listen at start-up for fan sounds. A whistling, rattling, loud grinding or complete silence when the boiler tries to start can indicate a faulty or seized fan.

5. Check visible plumbing for signs of water leak or accumulated condensate near the boiler base, which can affect fan or sensor electronics. Check system pressure and top up to the recommended 1–1.5 bar if it is low, then reset and observe.

Basic diagnostic steps (do not open the boiler casing or touch internal parts):

1. After isolating power, visually inspect any external wiring or plugs to the boiler that are accessible (e.g., terminal blocks behind an easy-access cover) for loose connectors or obvious damage. Reconnect only if accessible and safe—do not probe internal wiring.

2. If the boiler has an action log or display with further fault detail (e.g., fan calibration error, flue thermistor fault), record that information for the engineer. It helps pinpoint whether the fault is fan motor, sensor, or wiring/PCB related.

3. If the problem is intermittent and cleared by a reset, monitor closely. If it returns, log when and how often it happens and any noises or conditions (cold start, after a power cut, heavy rain, etc.).

When to call a professional and what they will do:

- Call a Gas Safe registered engineer if the error persists after the single reset, if you find a blocked flue you cannot safely clear, if you hear mechanical noise from the fan, if there is visible water ingress, or if any internal wiring or component fault is suspected. Do not attempt internal repairs, fan replacement, wiring fixes, or combustion testing yourself.

- The engineer will isolate and safely inspect the boiler internals, test and replace the fan motor or fan speed sensor if required, check flue thermistor and flue integrity, test the PCB and wiring for bad connections, check for water damage, and perform combustion and carbon monoxide checks after repair.

- If a gas or CO risk is suspected, the engineer will take immediate safety steps and may leave the boiler turned off until it is safe.

Final notes:

- A single reset can clear temporary faults, but repeated lockouts mean professional diagnosis is required. Do not repeatedly reset a locked-out boiler.

- Always use a Gas Safe registered engineer for gas appliance repairs and safety testing. Provide the engineer with the exact fault display and any observations you made (sounds, leaks, frequency) to speed diagnosis.