Error D.3

Safety precautions:

- Do not open the boiler casing or attempt electrical/gas repairs unless you are a qualified, Gas Safe registered engineer. Gas and electrical work is hazardous.

- Before any basic user checks, turn off the boiler using the on/off control or the isolator switch. If you remove power for a reset, wait one minute before restoring power.

- Keep the area dry and avoid touching internal components. If you smell gas, leave the property and call the emergency gas number.

Initial homeowner checks you can safely do:

1) Record the exact error and when it appears (only on hot water demand, continuously, after a power cut, etc.). This information helps the engineer.

2) Reset the boiler: switch off at the user control, wait 30–60 seconds, then switch on again or use the boiler’s reset function. Note whether D.3 returns immediately or only under certain conditions.

3) Check boiler pressure on the display. If pressure is very low (typically below ~1.0 bar) the boiler can behave erratically. If you are familiar with your system and it is safe to do so, top up the system pressure following the boiler manual instructions. Do not over-pressurise (avoid >2.5–3.0 bar). If you are unsure, leave this to an engineer.

4) Confirm power and gas supply: check other gas appliances to ensure the gas supply is present and check that the electrical supply and any fused spurs to the boiler are intact.

5) Check any external cylinder thermostat, programmer or room controller: ensure they are configured correctly and not commanding the boiler off or to an extremely low temperature.

6) Look for obvious signs of water ingress or loose wiring at the external terminals (visible without opening the boiler). Do not probe connectors inside the casing.

Specific diagnostic and repair steps for a qualified engineer (what to expect when a pro attends):

- Isolate gas and electrical supplies and open the boiler casing to inspect wiring and connectors.

- Visually inspect the flow/hot water NTC plug and wiring for corrosion, water damage, chafing or a disconnected/loose plug; reseat connectors and dry/replace any corroded plugs.

- Measure the NTC resistance with a meter at ambient temperature and compare to the expected value for that sensor (typical flow NTCS are around 10–12 kΩ at 20 °C, but the engineer will use Vaillant specifications). Check for open-circuit or short-circuit conditions and for wiring continuity back to the PCB.

- If the sensor is defective or wiring is faulty the engineer will replace the NTC or repair/replace the wiring harness as required.

- If the sensor and wiring check out, test the PCB input for correct readings and check for intermittent connector faults or PCB faults; replace PCB only if proven faulty.

- If the fault is related to temperature spread, pump performance or flow issues, the engineer will check the heating pump, valves, and system circulation and purge or repair as necessary. They may also check the cylinder thermostat and associated sensors on a regular (indirect) system.

- After repairs the engineer will clear the fault code, run the boiler through hot water demand cycles and verify stable temperature readings and normal operation.

When to call a professional:

- If the D.3 code persists after a reset and the basic checks above, switch the boiler off and call a Gas Safe registered boiler engineer. Any internal electrical tests, sensor replacements, wiring repairs or changes to gas components must be done by a qualified engineer. Provide the engineer with the exact error code, the circumstances under which it appears and any steps you have already taken.

Note: Do not attempt to replace sensors, open the boiler or work on wiring or gas components yourself. These repairs require specialist knowledge, correct parts and safe testing procedures.