Error D.3

Safety precautions:

- If you smell gas, stop immediately, turn off the gas supply if safe to do so, ventilate the area and call the gas emergency number and a Gas Safe engineer — do not try to fix the boiler.

- Before doing any inspections, switch the boiler off at the mains and isolate electrical supply if you will be accessing anything inside the casing. If you are not competent, do not open the casing.

- Do not attempt to repair or replace internal components (sensors, wiring, PCB, gas or combustion parts) yourself. Only a Gas Safe registered engineer should work on gas appliances.

Initial homeowner checks you can do (safe, external checks):

1. Record the error: note the exact code (D.3) and any additional messages or codes on the display.

2. Basic reset: try switching the boiler off at the programmer/boiler and then back on (or use the boiler’s reset function) and see if the fault clears. If it returns immediately or frequently, further diagnostics are required.

3. Check boiler display and behaviour: try calling for hot water (open a hot tap). Note whether the boiler attempts to fire and whether the error appears when hot water is requested.

4. Check system pressure: ensure the heating system pressure is within the normal range (usually around 1.0–1.5 bar on many boilers). Very low pressure can cause other faults; top up only following the manufacturer instructions.

5. Visual check external to the boiler: with power isolated, look for obvious signs of damage, water ingress, corrosion or loose external wiring to the boiler and any visible connectors. Do not open the boiler casing unless qualified.

6. Check service valves: make sure the pipework service valves on the heating flow and return are open (T-handles in line with the pipe). Blocked or closed valves can cause temperature or circulation faults.

If the basic checks do not clear the fault, call a Gas Safe engineer. Professional diagnostic steps they will take (for information):

- Power off and remove the boiler cover to inspect the flow NTC thermistor, return NTC and their plug connections to the main PCB for loose plugs, corrosion or water damage.

- Measure the NTC resistances with a multimeter at known temperatures (typical reference: around 12 kΩ at 20°C for many Vaillant NTCs) to confirm open circuit, short circuit or incorrect values.

- Test wiring harness continuity and insulation to check for short circuits, intermittent breaks or chafing against metalwork. Inspect connector pins and PCB sockets for damage.

- Confirm sensors are fitted in the correct positions on the flow/return pipes (not both on the same pipe) and seated correctly in the sensor pockets or clips.

- Check for related causes that can mimic sensor faults: pump operation, blockages, airlocks, closed isolation valves, high levels of debris causing poor circulation, and correct operation of the diverter valve if applicable.

- Inspect flue temperature limiter (STB) and other safety devices if over-temperature conditions are suspected, and check for sooting or flue issues that could lead to related faults.

- Replace defective NTC(s) or wiring harness/plug if faulty and test the system. Re-test boiler operation and clear any stored fault codes.

Final note and recommendation:

- Because D.3 relates to temperature sensing and wiring and can involve internal electronics and gas safety systems, the correct fix usually requires specialised testing and replacement of parts by a Gas Safe registered engineer. Arrange a professional service call rather than attempting internal repairs yourself. When you contact an engineer, give them the exact fault code (D.3), any other displayed codes, and details of what you observed during your checks to speed up diagnosis.