Error D.96

Safety precautions:

- If you smell gas or suspect a gas leak, leave the property immediately and call the emergency gas number. Do not operate electrical switches or the boiler.

- If you are not a qualified engineer do not remove the outer casing, work on the gas supply, or attempt PCB/electrical repairs. Isolate mains power before any visual checks inside the casing and only open the casing if you are competent and it is safe/allowed by the manufacturer.

Initial checks a homeowner can do:

1) Note exactly what the display shows and take a photo of the code and any accompanying messages or other fault codes.

2) Check whether the boiler is still producing heating or hot water. If it is, D.96 is likely informational.

3) Check boiler pressure on the gauge; if pressure is below 1 bar top up the system using the filling loop per the manual (only if you know how).

4) Check gas and electrical supply: confirm other gas appliances work and that the boiler has power (mains switched on, no tripped RCD/blown fuse).

5) Check for other obvious issues: frozen condensate pipe in cold weather, blocked flue outlet, or external controls/thermostat settings that could stop operation.

Specific diagnostic and practical steps:

1) Simple reset: press the boiler reset button or switch the boiler off at the mains isolation, wait about 30 seconds and power back on. Note whether D.96 returns. A one-off appearance that clears after reset is usually not critical.

2) If the code returns, check the boiler menu/user display for an indication that the unit is in service/installer mode or that parameters were reset. Do not change installer parameters unless you are a qualified engineer.

3) Record any other fault codes shown alongside D.96. If other fault codes are present, follow guidance for those codes or include them when you contact a professional.

4) If the boiler is not working (no heat/hot water) despite D.96, and you’ve confirmed gas and power are present, do not attempt internal diagnostics beyond visual checks. Photograph the display and any visible wiring/connector issues and avoid toggling internal connections.

5) If you (or a previous engineer) recently performed an update or parameter change, inform the attending engineer—D.96 can indicate a factory/default parameter state that may need reconfiguration.

When to call a professional:

- If the code persists after a reset.

- If the boiler will not produce heat or hot water.

- If there are other fault codes present or any signs of electrical/electronic failure.

- If you are unsure whether parameters have been changed or if you see service/installer menu settings.

Provide the engineer with: the exact code (D.96), any other displayed codes, photos of the display, and a description of recent events (power cuts, service visits, firmware updates, etc.). A Gas Safe registered engineer or Vaillant service will be able to check configuration parameters, run diagnostics on the PCB and sensors, and restore the correct factory/operational settings safely. Do not attempt gas or PCB repairs yourself.