Error D.91

Safety precautions:

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

- Do not remove sealed covers or touch gas or combustion components. If you are unsure, stop and call a Gas Safe engineer.

- Isolate electrical supply to the boiler before opening the casing for any inspection of wiring. If you are not trained, do not open the boiler.

Initial homeowner checks (safe, non-intrusive):

1. Note if any other fault codes are shown on the display. Multiple codes can change the priority of the diagnosis.

2. If the boiler has been recently worked on or an external controller/probe was added, confirm that work was completed correctly and the installer set the boiler parameters to expect the external probe.

3. Visually inspect any visible external probe cable and its route for obvious damage, crushing, or disconnected plugs. If the probe or cable is outside (immersion sleeve, external pipe) check for obvious water damage or disconnection at the end you can access safely.

4. Try a simple reset: press the boiler fault-clear/reset button (or switch the boiler off at the isolator, wait 1 minute, then switch on). If D.91 clears and does not return, monitor for normal operation. If the code returns, further checks are needed.

Basic diagnostic steps (only if you are competent and safe to do them):

1. Check the boiler display for any live temperature reading that relates to the external probe. If the display shows a temperature for the external probe, compare it to the actual room or cylinder temperature. Large differences indicate a bad probe or wrong placement.

2. If you have a multimeter and the boiler wiring is accessible and you are comfortable isolating power first, you can check the probe wiring continuity. Isolate electrical supply, open a service panel only if instructed in the manual, and measure resistance across the probe terminals. Many Vaillant temperature probes are negative temperature coefficient (NTC) sensors — they will show a finite resistance (not infinite) at room temperature; an open circuit (infinite resistance) or a near-short (very low ohms) indicates a wiring or sensor fault. If you do not know how to do this safely, stop and call an engineer.

3. If the probe is removable (external cylinder probe or outdoor probe) and accessible, and you are confident, disconnect it and inspect the connector. Reconnect firmly. If the boiler then reports no external probe but runs normally, the probe or cable is likely faulty or the boiler parameters need updating.

4. Check any external controller or thermostat settings. If a controller is set to use an external probe but no probe is connected, or the boiler parameters have been changed, that mismatch can trigger the message. Revert to the known working settings or consult the controller manual.

If these checks do not resolve the issue or if you see signs of wiring damage, repeated D.91 after reset, or other F-codes appearing:

- Do not continue to repeatedly reset the boiler. Record the exact code and any additional messages and call a Gas Safe qualified engineer or Vaillant support. Tell them the boiler model, D.91, whether an external probe is fitted, and any other fault codes.

- An engineer will be able to safely test and replace the probe, repair/replace wiring, check the PCB and configuration parameters (e.g. whether the boiler is set to expect an external probe), and carry out any required commissioning and parameter coding.

Summary: D.91 indicates the boiler’s DCF/configuration status associated with a connected external probe. You can do some safe visual checks and a single reset, but diagnosing wiring, sensor replacement or changing boiler configuration requires a competent, Gas Safe qualified engineer or authorised Vaillant technician. Keep a note of the code and any other symptoms to give to the engineer.