Overview
D.64 on a Vaillant EcoTEC Plus regular boiler is a diagnostic code indicating the control has detected average ignition times that are outside expected parameters. In plain terms, the boiler is taking longer than normal to ignite the burner or the ignition process is not behaving consistently. This can be an early warning of ignition- or gas-supply-related problems such as a weak spark, degraded ignition electrode or lead, incorrect gas pressure, air in the gas line, or problems with the gas valve or flame sensing circuitry. Severity ranges from moderate to high depending on the underlying cause. If ignition is only occasionally slow you may see intermittent lockouts or temporary faults; if ignition repeatedly fails or is much slower than normal the boiler can go into safety lockout and will not provide heating or hot water. Because the ignition and gas circuits are safety-critical and involve live electrical components and flammable gas, most diagnosis and all repairs beyond basic checks should be carried out by a qualified Gas Safe engineer. Simple homeowner checks can sometimes resolve temporary causes, but do not attempt to access ignition electrodes, gas valves, or internal wiring yourself.
Possible Cause: Average ignition times
Troubleshooting Steps
Safety precautions:
1. If you smell gas do not touch the boiler, do not operate electrical switches, open windows, leave the property and call the gas emergency number immediately.
2. Isolate electrical power to the boiler only if directed by the user manual and if you are confident doing so; otherwise leave power on for diagnostic displays so you can note error codes. Do not open the boiler casing or attempt internal repairs.
3. Never attempt to replace or adjust the gas valve, ignition electrodes, or PCB yourself. These tasks require a Gas Safe qualified engineer.
Initial homeowner checks you can do safely:
1. Note and record the exact error code (D.64) and any other F or D codes displayed, and when they occur (on boiler start, during DHW, after flame loss, etc.). This information helps the engineer.
2. Check gas supply: verify other gas appliances (hob, gas fire) are working. If no other appliances work, contact your gas supplier.
3. Check mains power to the boiler and the display. If there has been a power cut, a reset may be required.
4. Check boiler pressure on the gauge. If pressure is well below 1.0 bar, follow the boiler manual to top up the system pressure using the filling loop, then reset the boiler if appropriate.
5. Check external vents and the air intake to the boiler for obvious blockages (bird nests, snow, debris). Also check the condensate pipe for frost or blockages if it’s been very cold.
6. Attempt a controlled reset using the boiler’s reset procedure (press and hold the reset button per manual) and observe if the boiler tries to ignite and whether the code returns. Do not repeat resets if the code persists; this will not fix a hardware fault and could mask the problem.
Specific diagnostic and fix steps (for the engineer or to prepare information for the engineer):
1. Record ignition behaviour: whether ignition is slow every start, intermittent, or only after flame loss. Note any visible flame instability or prolonged sparking before flame establishment.
2. Engineer checks: inspect ignition electrode and lead for damage, correct gap and correct positioning; check for carbon/black tracking on the lead which can leak current and delay ignition.
3. Check the ignition transformer / spark generator operation and measure ignition voltage; replace if defective.
4. Measure inlet gas pressure and working pressure to the gas valve; check for air in the gas line, a faulty regulator, or a partly closed isolator cock.
5. Test gas valve assembly operation and coil resistance; verify gas valve timing and calibration stored in diagnostic parameters (e.g. D.052/D.182) if applicable.
6. Check flame sensing/ionisation circuit and wiring for continuity and correct earthing; an unstable or missing flame signal can look like slow ignition.
7. Inspect the burner for deposits, obstructions or corrosion that could slow ignition and clean if required.
8. Check for flue or air intake blockage or flue gas recirculation which can affect combustion and ignition.
9. Verify that condensate is not flooding the combustion chamber (blocked condensate) and check condensate pump if fitted.
10. Check PCB and wiring harness for intermittent faults, moisture or damage; replace PCB only after ruling out sensors, ignition and gas valve faults.
11. After repairs, run multiple start cycles and a flue gas analysis if available to confirm correct combustion and safe operation. Clear stored fault codes and monitor for recurrence.
When to call a professional and final notes:
1. If the initial safe checks above do not clear the fault, or if the fault recurs, stop attempting resets and call a Gas Safe qualified engineer. Ignition and gas valve work is not suitable for DIY.
2. Provide the engineer with the recorded error codes, the exact behaviour you observed (when the fault occurs), and results of the basic checks you performed. This will speed up diagnosis.
3. Do not continue to use the boiler if it repeatedly locks out or if you detect gas smells, persistent flame instability, or signs of overheating. Turn off and isolate per the manual and wait for the engineer.
This guidance is for information only. Always follow the boiler manufacturer’s instructions and use a Gas Safe registered engineer for any repairs involving gas, ignition, or internal electrical components.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Plus Regular Gas Boiler.