Error D.47

Safety precautions:

1. If you smell gas or suspect a gas leak, leave the property immediately and contact the gas emergency service. Do not operate the boiler or any electrical switches.

2. Before doing any inspections, switch the boiler off at the external isolator and, if possible, at the fused spur. Only perform visual checks outside the boiler casing unless you are a qualified engineer.

3. Do not open the boiler casing or touch internal components (gas, PCB, wiring) unless you are Gas Safe qualified.

Initial checks a homeowner can do:

1. Note exactly what the display shows with D.47: does it show a temperature value, a dash, or an obvious error alongside D.47? Write down any other codes that appear.

2. Confirm your system is configured for weather compensation and that an outside sensor/probe is fitted. The outside sensor is usually a small probe fixed to an external wall or wired back to a controller box.

3. Locate the external probe and inspect it visually for obvious damage, water ingress, loose mounting or crushed cable. Also check along the visible cable run for cuts or chewing by rodents.

4. Compare the outside temperature shown on the boiler/controller with a reliable reference (phone weather app or a thermometer). If the boiler reading is close to the reference, the sensor is likely working.

5. Try a simple reset: on many Vaillant controllers press and hold the power/reset button for about five seconds to reset the boiler. If D.47 disappears or the value updates and remains correct, the issue may have been transient.

Specific diagnostic and (non-invasive) fix steps:

1. If the boiler shows an implausible outside temperature (very high/low or blank) or D.47 persists after reset, the likely causes are a faulty outside probe, a damaged cable, a loose connector or a problem with the controller/PCB input.

2. If the external probe has a visible connector on a controller box or an external junction that is accessible without opening the boiler, you can power down the boiler and carefully reseat that connector. Restore power and check the display again.

3. If you are competent with basic electrical safety and have experience, you can measure the probe wiring for continuity with the boiler isolated from mains. Only do this if you know what you are doing and have isolated power. If you are not competent, stop here and call a professional.

4. If the probe or cable is obviously damaged, do not attempt to repair it yourself unless it is an external low-voltage cable and you are qualified; replacement is normally the correct action.

When to call a professional:

1. Call a Gas Safe–registered engineer or Vaillant-qualified technician if D.47 remains after the simple checks, if the outside probe or wiring needs replacing, if any connectors inside the boiler appear to be the problem, or if other fault codes appear at the same time.

2. The engineer will perform safe isolation, test the thermistor/NTC characteristics, check wiring continuity to the PCB, inspect the controller and eBUS connections, and replace the external sensor or harness if necessary. They will also check that weather compensation settings are correct and verify the boiler modulates correctly after repair.

3. When you call a professional, give them the boiler model, serial number, the D.47 diagnostic and any other codes or symptoms you have observed, and describe what checks you have already performed.

Important: do not attempt to work on gas valves, the burner, the PCB or internal wiring yourself. Any internal electrical or parts replacement must be carried out by a qualified engineer to ensure safety and maintain warranty/insurance cover.