Error 4 flashes red

Safety precautions:

1. If you smell gas or suspect a carbon monoxide leak (headache, nausea, flu-like symptoms, excessive soot, pilot light behaving oddly), turn off the boiler, ventilate the area, leave the property and call the gas emergency number immediately. Do not attempt repairs.

2. Before doing any checks, isolate electrical power to the boiler at the fuse or isolation switch. Do not remove covers or access internal components unless you are a qualified engineer.

3. Keep children and pets away from the boiler while you inspect it.

Initial checks a homeowner can safely do:

1. Check the boiler display and reset: press and hold the reset button for 3–5 seconds and allow the boiler to attempt a restart. If the error clears and the boiler runs normally for several cycles, monitor it—if it returns call an engineer.

2. Verify mains electricity and gas supply: make sure the boiler display is powered, check other electrical appliances and ensure the gas supply is on (try another gas appliance such as a hob). If gas or power is off, restore supply and reset the boiler.

3. Inspect the external flue terminal: look outside to ensure the flue outlet is not blocked by birds, nests, leaves, debris or snow/ice. Remove obvious obstructions safely from ground level if possible. Do not climb or force access to the flue.

4. Listen during a reset/start attempt: if you hear the fan spin briefly and then stop or hear unusual noises (rattling, grinding, whistling), note this—mechanical fan failure is likely.

Specific diagnostic and basic fix steps (only non-invasive steps):

1. After isolating power, visually inspect accessible wiring and connectors to the boiler (external terminals and plug-in connectors visible without removing covers) to check for loose or obviously damaged connections. Reconnect any loose plug connectors if they are accessible and easy to reach. Restore power and retry reset.

2. If the boiler reports the fault only on cold starts or in particular weather, check for a blocked or frozen condensate pipe or flue obstruction and clear as appropriate (thaw frozen condensate with warm water—do not use boiling water). If you suspect a frozen condensate pipe, follow manufacturer guidance and then reset.

3. If the fan appears to run but the error persists, this can indicate a fan calibration error or a fault in the combustion control unit (PU) or PCB. There is no safe homeowner procedure to recalibrate the fan or replace internal control modules—this requires diagnostic tools and replacement parts.

4. If the flue and visible wiring are clear and a reset does not clear the fault, do not attempt to dismantle the fan, burner, or PCB. These are live, gas-containing components and require a Gas Safe registered engineer.

When to call a professional and what to tell them:

1. Call a Gas Safe registered engineer if the fault persists after simple checks and a reset, if you hear mechanical noise from the fan, if the flue terminal looks damaged, or if there are repeated lockouts.

2. Tell the engineer the exact error (4 red flashes / fan fault), what you have already checked (reset, flue clear, power/gas supply checked, any noises heard), and any intermittent behaviour. Mention if the boiler shows the fault immediately on start or after running a short time.

3. Expected professional actions: measure fan operation and calibration, check fan motor and impeller condition, inspect fan wiring and connector integrity, inspect flue and flue sensor, and test/replace the combustion control unit (PU) or PCB if required. Only a qualified engineer should carry out these repairs and re-calibration.

Final note: because a faulty fan affects combustion safety, do not continue to use the boiler if the fault persists. Arrange for a Gas Safe engineer to inspect and repair the appliance rather than attempting internal repairs yourself.