Error D.65

Safety precautions:

1) If you smell gas, stop using any electrical switches, extinguish naked flames, ventilate the property, leave immediately and call the gas emergency number or your gas supplier. Do not try to fix the boiler yourself.

2) If the boiler shows fault D.65, do not repeatedly attempt to run it without investigation — repeated ignition attempts can mask the underlying problem and may be unsafe. Call a qualified engineer if basic checks don't resolve it.

3) Isolate electrical power to the appliance before attempting any internal inspection and never open the boiler casing unless you are qualified.

Initial checks a homeowner can safely do:

1) Note the exact behaviour and any additional fault codes or messages. Take a photo of the display if you can — this helps the engineer.

2) Try a single reset: press and hold the reset button (or the controller’s reset) for up to 10 seconds and see if the boiler lights. If it relights and runs normally for a while, note how long and whether it trips again. Do not attempt continuous resets.

3) Check the gas supply: confirm other gas appliances are working. Ensure the gas isolation cock to the boiler is open.

4) Check mains power and fuses: ensure the boiler has electrical power and no tripped RCD/fuse in your fusebox.

5) Check boiler pressure: confirm system pressure is in the normal range (usually around 1–1.5 bar on Vaillant units). If pressure is very low, top up per the user manual or call an engineer.

6) Check external vents and flue: make sure the air intake and terminal flue outside are not blocked by debris, bird nests, snow or vegetation.

7) Check the condensate discharge (in cold weather): if condensate pipe or trap is frozen or blocked, it can prevent ignition. Thaw external condensate pipe gently with warm (not boiling) water or seek a pro if unsure.

8) Check visible service valves: ensure heating flow and return service valves are open (T-handles aligned with pipe direction).

Diagnostic and fix steps an engineer will typically perform (for information only):

1) Read fault history and run live diagnostics with service menu. Confirm D.65 details and any linked faults (flow/return temperature errors, mass flow sensor, F-codes etc.).

2) Check ignition sequence: verify spark at the ignition electrode, electrode gap and condition, ignition lead and ignition transformer output, and that the electrode is positioned correctly and not shorting to the burner.

3) Verify flame detection/ionisation circuit: measure ionisation voltage/signal to ensure the flame sensor picks up the flame. Inspect wiring harness and connectors for corrosion or moisture.

4) Check gas supply to the valve: measure inlet gas pressure and gas valve outlet pressures during ignition attempt, confirm gas valve coil resistances and operation. Inspect for air in gas supply (common on commissioning) or low supply pressure.

5) Inspect gas valve assembly and mass flow sensor: test for correct actuation and feedback, check wiring to PCB.

6) Inspect fan and flue/air route: confirm fan starts and provides correct flow, check for flue blockages, recirculation or blocked air intake.

7) Check pumps, flow/return sensors and circulation: a lack of water flow or an over-temperature safety cut-out (SCO) can shut the gas off; measure NTC sensors, pump operation, and check for air in system, blocked diverter valve or faulty expansion vessel.

8) Inspect condensate trap and discharge, and look for condensate in the combustion chamber (blocked condensate can cause ignition problems).

9) If electronic faults suspected, test PCB outputs and inputs; replace PCB only after validating sensors and actuators.

When to call a professional:

1) If the single reset does not clear the fault or the fault returns, call a Gas Safe registered engineer. Do not open or replace ignition/electrical or gas components yourself.

2) If any of the homeowner checks identify a gas supply problem, smell of gas, persistent flue/vent blockage, frozen condensate or repeated ignition failures, call your gas supplier or a Gas Safe engineer.

3) Use a Vaillant-authorised service engineer where possible; you can contact Vaillant customer service for guidance or locate a registered installer.

Final notes:

1) D.65 indicates an ignition/combustion safety issue. It prevents the boiler running until corrected. Basic checks listed above may resolve simple supply or venting issues, but most root causes require qualified diagnostic equipment and replacement parts.

2) Document what you tried and any other displayed fault codes; this will speed up the engineer’s diagnosis.

3) Never attempt internal electrical or gas repairs unless you are suitably qualified and registered to work on gas appliances.