Error D.64

Safety first:

1) If you smell gas: do not operate electrical switches, do not use phones inside the building. Evacuate everyone, call your gas emergency number and a Gas Safe engineer from a safe location.

2) If you suspect carbon monoxide (unusual fumes, headache, dizziness): get fresh air, turn off the boiler at the isolator if safe to do so, and call a qualified engineer or emergency services.

3) Do not remove the boiler casing or touch gas or electrical components. Only carry out non-intrusive checks listed below.

Initial homeowner checks (safe, simple steps):

1) Note the exact error code, how frequently it appears and whether it follows any events (power cut, cold night, recent servicing). Take a photo of the display for the engineer.

2) Reset the boiler once: press and hold the reset button (or use the boiler's reset procedure) for up to 10 seconds. If the code returns immediately or repeatedly, stop and call an engineer.

3) Check boiler pressure: view the pressure gauge; normal operating pressure is typically about 1.0–1.5 bar. If pressure is very low (<0.8 bar) top up the heating system following the boiler handbook. Low water pressure can cause safety shutdowns; do not continue if the boiler still faults after correct pressure is restored.

4) Check gas supply: confirm other gas appliances (hob, gas fire) are working. If no gas to the house, contact your gas supplier. If there’s intermittent gas supply, the boiler may fail to ignite.

5) Check condensate discharge: in freezing weather the external condensate pipe can freeze and block. Thaw frozen condensate pipes with warm (not boiling) water. A blocked condensate trap can prevent ignition/operation.

6) Check vents and flue/air intake: make sure air vents, grille and the boiler’s air intake and flue terminal are not blocked by debris, nests, snow or leaves.

7) Ensure mains electrical supply and external switches/fuses to the boiler are on.

When to call a professional (must-do if checks above don’t resolve it):

1) If the boiler locks out again after reset, or the D.64 returns intermittently.

2) If you observed ignition noise without flame, erratic flame behaviour or frequent flame loss.

3) Any signs of gas smell, persistent combustion smell, soot or corrosion around the appliance.

What a Gas Safe engineer will check and do (professional diagnostic and repair steps):

1) Capture live logs and check ignition timing and ionisation/flame signal values using the boiler’s service menu and instruments.

2) Check for correct gas inlet pressure and steady supply; inspect the gas meter/regulator if needed; purge air from gas line if required.

3) Inspect ignition components: verify spark from the ignition transformer, check condition and position/gap of the ignition electrode, check ignition lead for damage or tracking, and replace components if defective.

4) Measure ionisation/flame sensor signal and earth/earthing integrity; clean or replace electrode/ionisation lead if signal is weak or unstable.

5) Test gas valve operation and coil resistance (typical values around the manufacturer specification) and check stored gas valve offsets/parameters. Replace gas valve if faulty.

6) Inspect fan and flue/air route for blockages or recirculation problems and test fan speed/operation.

7) Check PCB for fault codes and potential moisture or corrosion; replace PCB if defective and after confirming root cause.

8) Check flow and return NTC temperature sensors and wiring for correct resistances and possible short/open circuits, because incorrect sensor readings can cause safety intervention.

9) Verify pump operation, flow rate and diverter valve operation; clear airlocks and ensure correct circulation so temperatures remain within safe limits.

10) Check condensate system and clear any blockages; ensure condensate pump (if fitted) operates correctly.

11) After repairs, run the boiler through several ignition cycles and monitor flame/ignition timing and safety cut-outs to ensure the issue is resolved.

Final notes:

- Because D.64 relates to ignition/flame behaviour and may involve gas, ignition parts or the control electronics, you should call a Gas Safe registered engineer if basic homeowner checks do not fix the fault. Do not attempt internal repairs yourself. Provide the engineer with the error code, how often it happens, and any recent events (freezing weather, power cuts, recent servicing) to speed diagnosis.