Error D.34

Safety precautions:

1) If you smell gas, leave the property immediately and call the gas emergency number; do not operate electrical switches or the boiler. 2) Isolate electrical power to the boiler before opening the case if you are going to do any physical inspections. 3) Do not attempt to access or repair gas-carrying parts, the burner, or electrical components unless you are a Gas Safe registered engineer. 4) Take photos of the fault code and display for the engineer and note when the fault appears (continuous or intermittent).

Initial homeowner checks you can safely do:

1) Record the exact code and any additional messages shown on the display. 2) Try a boiler reset using the reset button or via the control panel; note whether the code returns immediately or after a short run. 3) Visually inspect the external flue terminal and air intake for obvious blockages (birds' nests, debris, snow/ice) and remove any external obstruction if it is safe to do so. 4) Listen when the boiler starts: do you hear the fan spin, or any unusual noises (grinding, wobble)? 5) Check condensate discharge for blockages and ensure external condensate pipe is not frozen or blocked in cold weather. 6) Confirm the boiler has correct mains power and the gas supply is on (only if you are sure there is no gas leak).

More specific diagnostic checks (recommended only if you are competent with basic electrical isolation; otherwise call a pro):

1) After isolating electrical power, open the boiler cover and visually inspect the fan wiring connector for loose pins, corrosion or damage; reseat the connector if it is accessible and intact. 2) With power reinstated, watch (or record) the boiler start sequence: the fan should start before ignition—if it does not spin, or spins and stalls, that points to fan motor failure or blockage. 3) Note whether the fan runs but the control still flags a speed error—this suggests the hall sensor or feedback circuit is at fault (sensor wiring, sensor on motor, or PCB input). 4) Do not attempt resistance or live testing of gas/combustion circuits unless you are a qualified engineer; incorrect testing can be dangerous and may damage the PCB.

When to call a professional and what to tell them:

1) Call a Gas Safe / Vaillant-authorised engineer if the code returns after a reset, if the fan does not spin, if you hear abnormal noises from the fan, or if you see damaged wiring. 2) Provide the engineer with the boiler model, serial number, the exact fault code (D.34), a description of what you observed (did the fan run? was there a blockage? did reset temporarily clear it?), and any photos of the display and flue terminal. 3) The engineer will properly isolate the appliance, remove and test the fan assembly, check the hall sensor and wiring harness, inspect the flue for restriction, and if required test the PCB inputs. They will replace the faulty part(s) (fan motor, hall sensor, wiring, or PCB) as appropriate and run combustion and safety checks.

Final notes:

1) Because the fan and its control are critical to safe combustion and flue evacuation, avoid prolonged attempts to run the boiler with a recurring D.34 fault. 2) If in doubt, turn the boiler off and arrange qualified service. 3) Keep a record of the fault occurrences and any actions taken—this helps the engineer diagnose intermittent issues.