Error D.34

Safety first

- If the boiler is showing any active fault, especially gas-related or flame/ignition faults, do not attempt internal repairs. Isolate power to the boiler before touching anything externally. Do not open the casing or work on gas/electrical parts unless you are a qualified, registered engineer.

Initial homeowner checks (safe, simple steps)

1. Check the boiler display for any accompanying F-codes (for example F.32) or lockout messages. Note any other codes and when they appeared.

2. Check the boiler is powered and the mains fuse or isolation switch is on. If the boiler has been powered down recently, try a simple reset: use the boiler reset button or switch the boiler off at the isolation switch for 30–60 seconds, then switch it back on. See if the code returns.

3. Listen to the boiler when it is trying to fire: can you hear the fan spin up? Any unusual rattling, grinding or absence of fan noise is a clear sign of a problem.

4. Check that external flue terminals, air vents and outside termination are not blocked by debris, nests, snow or heavy bird activity. Make sure the area around the boiler has adequate ventilation and the condensate pipe is not frozen or blocked.

5. Check water pressure and other routine items on the display—low system pressure or unrelated faults can sometimes mask other issues. Top up pressure only following the manual if needed.

Viewing the diagnostic fan speed value (information only)

1. Wake the display and enter the diagnostics as per Vaillant user guidance: press the information (i) button and the plus button together to enter the diagnostic range, then scroll to D.034 (do this only to view the value; do not change settings unless you are an installer).

2. Note the fan speed value shown while the boiler is attempting to run (or when hot water/heating is demanded). A zero value while the boiler is trying to fire, or a value that stays very low or fluctuates widely, indicates the fan is not running correctly.

3. Also check diagnostic history and any related F-codes that appear immediately after the D.34 reading.

What the values mean and simple next steps

- If D.34 shows a normal non-zero value and the boiler operates normally: the diagnostic is just reporting a healthy fan speed; no action needed beyond routine servicing.

- If D.34 shows zero or a very low/unstable value and the boiler has failed to ignite or has thrown a fan-related F-code: perform the safe checks above (reset, clear external blockages, verify mains power). If the fan still does not run or the code persists, do not try to disassemble the fan or the boiler internals.

When to call a professional

- Call a Gas Safe registered heating engineer or Vaillant-authorised service if the diagnostic shows a faulty fan speed (zero or abnormal) after simple resets and external checks, if you hear unusual fan noises, or if the boiler is locked out with a fan-related F-code. The engineer will safely test the fan motor, the fan wiring and connector, the control signals from the PCB, and the PCB itself; they will also verify flue and combustion performance.

Important notes

- Do not attempt to repair or replace the fan, fan wiring, fuses on the PCB or any gas components yourself. Work on these parts requires certification and correct parts.

- If this diagnostic repeatedly appears between services, arrange a prompt professional inspection to avoid repeated lockouts or unsafe operation. Annual service by a competent engineer helps prevent fan and combustion-related faults.