Error D.97

Safety precautions:

1. Do not open the boiler casing or attempt internal electrical, gas or flue repairs unless you are a Gas Safe registered engineer.

2. If you smell gas, evacuate the property, turn off the gas supply at the meter if safe to do so, and call the gas emergency number immediately. Do not operate electrical switches or the boiler.

3. Turn off power at the isolator or fuse if instructed, and only restore power when safe. Avoid repeated resets more than once or twice — repeated resets can mask a persistent fault and may cause further issues.

Initial checks a homeowner can safely do:

1. Note exactly what is displayed on the screen (D.97 and any other F or S codes). Take a photo of the display for reference.

2. Check whether heating and hot water are working normally. If everything operates normally and no F codes are present, D.97 may simply indicate diagnostics are active.

3. Check basic system status: gas supply to the property (are other gas appliances working?), boiler pressure on the gauge (normal range typically about 1.0–1.5 bar when cold), and visible error lights.

4. Check whether someone may have pressed buttons and entered the diagnostic menu (for example by pressing the information (i) and + buttons). If the boiler was recently serviced, an engineer may have left it in diagnostic mode.

How to safely exit the diagnostic mode and perform a basic reset:

1. If you or a previous user accidentally entered diagnostics, try exiting by pressing the back/return button on the user interface or by powering the boiler off and on: switch off at the boiler isolator or the mains for 30 seconds, then switch back on.

2. If the boiler has a reset button and the display advises a one-time reset, press reset once. Do not perform repeated resets more than twice.

3. After restart, check if D.97 is gone and whether heating/hot water function has returned to normal. If everything is normal and no fault codes remain, no further action may be needed.

If D.97 persists or is accompanied by fault codes or loss of function (what to record for the engineer):

1. Record or photograph the full display including any F or S codes, and note when the issue started, recent service work, power cuts or freezes, and any odd noises or loss of heating/hot water.

2. Do not change installer-level settings or alter diagnostic parameters. Installer menus can change safety or performance limits and should only be used by qualified engineers.

3. Call a Gas Safe registered Vaillant engineer and provide the model, serial number, the D.97 message and any other codes or symptoms you recorded. Mention whether the boiler locked out, shows flame or ignition faults, pump or pressure messages, or is operating incorrectly.

What a professional will likely check (do not attempt these unless qualified):

1. Use the diagnostic level to read stored values and error history, and interpret D codes in conjunction with F codes.

2. Check sensors (flow/return NTCs), wiring harness and plug connections, pressure sensors, pump operation, fan and flue for blockages, ignition and flame detection circuits, and the PCB for faults or software/DSN issues.

3. Test and replace faulty sensors, connectors or electronic modules as required and return the boiler to correct configuration and software ID if an invalid device specific number or configuration is present.

Final note: D.97 itself is a diagnostic state indicator and not a direct repair action. If the boiler is otherwise fine you can exit diagnostics and monitor. If D.97 is persistent, returns after reset, or appears alongside functional problems or F-codes, stop further DIY attempts and book a Gas Safe registered engineer to inspect and repair the boiler.