Error D.91

Safety precautions:

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

- Do not open the sealed parts of the boiler or attempt internal electrical or gas repairs unless you are a Gas Safe registered engineer. Live electronics and gas components are hazardous.

- You can safely perform basic external checks and a reset, but avoid touching internal PCBs, live wiring or gas components.

Initial homeowner checks you can do:

1. Note exactly when the fault appears and any accompanying fault codes (for example F.91, F.10, etc.). This helps the engineer.

2. Try a simple reset: switch the boiler off at the room thermostat or boiler power switch, wait 20–30 seconds, then switch back on. If the code clears and does not return, monitor for reappearance.

3. Check the boiler display and any external controllers or cylinder/stat wiring that are externally accessible (for example at a hot water cylinder head or actuator). Look for visibly unplugged connectors or obvious damage to cables. Only inspect connectors you can reach without removing the boiler casing.

4. Check the system pressure on the boiler gauge. Very low pressure can cause temperature control problems. If pressure is well below the normal range (typically below 1.0 bar), it may need repressurising; follow your boiler manual or call an engineer.

If the fault persists, call a Gas Safe registered engineer. Tell them you have a D.91 (DCF status with connected external probe) and supply any other fault codes and the circumstances when it appears. A safe engineer diagnosis will typically follow these steps:

1. Visual inspection: check all NTC sensor connectors, the external probe connection, and the wiring harness for chafing, pinched cables, corrosion or loose plugs. Inspect PCB plug connections for multiple plugs incorrectly seated.

2. Electrical tests: using a multimeter/ohmmeter the engineer will measure continuity and resistance of the external probe and flow/return NTCs. Vaillant NTC sensors are typically around 10 kΩ at 25°C (confirm the exact value for your model). The engineer will check for open circuits (infinite resistance), short circuits (very low resistance), and correct resistance vs temperature behaviour.

3. Connector and harness checks: test for mass shorts between sensor plug and boiler casing, inspect coded resistors and the wiring loom for damage. Re-seat and retest connectors.

4. System checks: if related codes indicate flow/return spread problems, the engineer will check pump operation, possible airlocks, blockages or debris in the system and ensure correct circulation. They may recommend flushing or pump service if flow is inadequate.

5. Component replacement: depending on results the engineer may replace the external probe (NTC), the cable/harness, the sensor plug, or, if necessary, the PCB or related electronics. They will also verify coding resistor values if relevant.

6. Final test: after repairs the engineer will clear fault history, cycle the boiler through heating and hot water modes, and confirm stable sensor readings and correct operation.

Important final notes:

- Because this fault relates to sensors and control electronics on a gas appliance, do not attempt internal repairs yourself. A Gas Safe registered engineer must carry out electrical measurements, component replacements and any work involving the boiler casing, gas or mains supply.

- Provide the engineer with the full fault history and any other codes shown (for example F.91, F.10 etc.) to speed diagnosis.

- If the fault appears intermittently after a reset, still arrange a professional check. Intermittent wiring faults or failing sensors can become complete failures or trigger unsafe conditions if left unaddressed.