Error F3

Safety precautions:

1) If you smell gas, leave the property immediately and call your gas emergency number. Do not operate light switches or the boiler. 2) Isolate electrical supply to the boiler only if you are comfortable doing so; otherwise leave isolation to a qualified engineer. 3) Never open the boiler casing, touch internal wiring, or attempt to repair gas components unless you are Gas Safe registered.

Initial homeowner checks you can safely do:

1) Reset the boiler: switch the boiler off at the programmer or isolator, wait 1-2 minutes, then switch back on and try to restart. Note whether F3 returns immediately, intermittently, or after running for a while. 2) Check the external flue termination for visible blockage, bird nests, debris, or physical damage. Clear only obvious external obstructions if safe to do so. 3) Listen for unusual fan noise before lockout (rattling, grinding or loud vibration) and note any recent noises, which suggest bearing or mechanical failure. 4) Look around the boiler for signs of leaks or condensation and any visible water on wiring or beneath the unit; water ingress can corrupt wiring or the PCB. 5) Check that the boiler was not recently moved or installed and that no pipework or wiring looks loose at accessible terminals; do not probe or undo connections.

What an engineer will do for diagnosis and repair (do not attempt these steps unless Gas Safe registered):

1) Verify safety and isolate gas and electrical supplies. 2) Remove the boiler casing to inspect the fan assembly, wiring harness and PCB. 3) Check the flue path internally and externally for restriction or damage that could cause abnormal fan loading. 4) Observe fan operation: confirm the fan spins freely by hand (with power isolated) and that bearings are not seized; check for blade damage. 5) Power up under controlled test conditions and measure fan supply and control signals with a multimeter/oscilloscope to confirm the fan receives the correct voltage/PWM control and that the PCB sees the expected feedback. 6) Check continuity and insulation of fan motor wiring and the connector to the PCB; inspect for corrosion, chafing, or water ingress. 7) Inspect the PCB for water damage, burnt components or bad solder joints; be aware that a damaged PCB can give misleading fault codes. 8) If bearings are worn, the fan assembly is usually replaced as a complete unit rather than attempting to rebuild bearings. 9) Replace faulty wiring, connectors, fan assembly or PCB as required. 10) Re-program or adjust fan speed if the fan was set incorrectly on installation and the model allows adjustment. 11) After repair, run a full ignition and flue-flow test, check combustion and CO/CO2 where appropriate, and confirm the F3 fault no longer appears over normal operating cycles.

Final notes:

1) Do not attempt internal electrical or gas repairs yourself. Any work on the fan, wiring, gas train or PCB must be completed by a Gas Safe registered engineer. 2) If you see signs of water ingress, leaking heat exchanger or pump faults, mention this to the engineer because moisture damage can affect multiple components. 3) When you call an engineer, tell them whether the fault is constant or intermittent, any abnormal noises before lockout, and whether you found any external flue obstruction or signs of leaks — these details speed diagnosis and safe repair.