Error D.71

Safety first:

1. If you smell gas, evacuate the property and call the gas emergency number immediately. Do not attempt any checks.

2. Before touching the boiler, switch off the electrical supply at the isolator and, if instructed by the manual, turn off the boiler’s gas supply. Only perform non-invasive checks that do not require opening the casing unless you are qualified.

3. Do not repeatedly reset the boiler. Repeated resets can mask faults and may damage components.

Initial homeowner checks (safe, non-invasive):

1. Note and photograph the exact error code(s) displayed (D.71 and any accompanying codes such as F.91, F.20 etc.). This helps the engineer.

2. Try a single reset using the boiler reset button and see if the code returns. If it returns immediately or after a short time, further checks are needed.

3. Check heating settings: lower the heating flow temperature from any maximum setting back to a typical value (e.g. 60–65°C) and see if the fault clears. While a high temperature setting alone rarely causes D.71, it is a quick check and harmless to try.

4. Check the boiler pressure on the display or gauge; top up to the normal range (typically ~1–1.5 bar) only if you know how and follow the manual. Very low pressure can cause other faults.

5. Ensure heating flow and return service valves (the T-handles on the pipework) are open and that any external isolation valves to the boiler are not closed.

6. Bleed radiators to remove any trapped air that could cause large temperature differentials or pump issues. Listen for unusual pump noise when the boiler tries to run.

7. Look for obvious signs: water leaks, damaged external wiring, or a blocked condensate drain or visible flue obstruction outside the appliance.

If the basic checks do not clear D.71, call a Gas Safe registered engineer. Tell them you have code D.71 and describe any other codes and the checks you performed. The engineer will perform these professional diagnostic steps:

1. Electrical and visual inspection: with the boiler isolated and the casing removed, check NTC sensor plugs and connectors for being unplugged, loose, corroded, or damaged. Inspect the wiring harness and PCB connector seating.

2. Measure sensor resistances: measure the heating flow NTC resistance (typical reference ~12 kΩ at 20°C for many Vaillant NTCs) and compare to expected values and the return sensor. Also check for shorts or open circuits in the harness using a continuity tester/multimeter.

3. Check for short circuits between flow and return sensors and to earth. Inspect for cable damage, chafing or moisture ingress.

4. Test the pump and system flow: confirm the pump runs correctly and that there are no air locks, blockages or closed valves causing abnormal temperature spreads between flow and return.

5. Check safety devices: examine the flue gas temperature limiter (STB) and clean or replace if sooted or faulty, and check for any over-temperature trigger or signs of overheating on the primary heat exchanger.

6. Check associated components: if the fault appears with DHW codes (e.g. F.91), the engineer will check domestic hot water/cylinder sensors and actoSTOR connections as needed.

7. Replace faulty parts as required: this may include the heating flow NTC sensor, the sensor cable/harness, connectors, or—if diagnostics indicate—main PCB or other components. After replacement, the engineer will fully test the system under load and clear the fault memory.

Important notes and when to call a professional:

- D.71 usually requires electrical testing and possible replacement of sensors, harnesses or PCB parts; these are not DIY tasks. Only a Gas Safe registered engineer should remove the boiler casing, work on gas or mains-connected electronics, or change components.

- When you call the engineer, provide the exact error code(s), the boiler model and serial if available, and the troubleshooting steps you already tried.

- Do not ignore the fault. A sensor or wiring failure can leave the boiler running incorrectly or not at all and can prevent essential safety interlocks from operating.

If you need assistance finding a Gas Safe engineer, contact Vaillant support or use the Gas Safe Register to locate a qualified installer in your area.