Error D.7

Safety first

- Do not open the sealed boiler casing or attempt internal electrical or gas repairs. If you are in any doubt, turn the boiler off and call a qualified engineer. Only perform the external checks described below.

- If you need to power the boiler down for any checks, use the boiler controls (press the power button) and isolate the external electricity supply if required. Do not isolate gas or attempt to work on gas connections yourself.

Initial homeowner checks (simple, safe checks you can do now)

1. Note the exact code and any other codes or messages on the display; take a photo of the display for the engineer if needed.

2. Check the boiler display menu for Comfort / Preheat and Target Flow Temperature settings. Comfort/preheat can cause warm start cycles; if Comfort is on, try turning it off and observe whether D.7 clears.

3. Check the target flow temperature setting: if it has been changed to an unusually high or low value, reset it to a normal value (many households use 50–65 °C for flow temperature, but use the value you normally had or the installer’s recommendation). Save/confirm changes on the display.

4. If the boiler has an external controller or room thermostat, verify the controller settings and that it is communicating correctly with the boiler. If a controller is present, the boiler may only allow certain maximums to be set from the controller rather than on the boiler panel.

5. Reset the boiler: press and hold the power/reset button for 5 seconds (or follow the reset instructions in your user manual). After reset, observe whether the D.7 code returns.

6. Check basic installation conditions: ensure the gas isolator valve (if externally accessible) is open and heating system service valves are open. Check boiler pressure on the front display; if pressure is very low (e.g. under about 1.0 bar) topping up may be needed for general operation (refer to your manual for the correct filling procedure). Note: topping up is for pressure issues and will not fix an internal sensor fault.

If the code persists after initial checks

1. Repeat configuration checks: restore default/known-good heating flow target and disable Comfort/preheat. If you cannot change some settings because they are locked or configured by an external controller or installer, note that and report it to the engineer.

2. Watch for any related fault codes (F20, F22, F75, F83, etc.). If D.7 is accompanied by other faults or the boiler locks out (won’t provide hot water or heating), do not continue troubleshooting yourself.

3. Avoid repeatedly resetting the boiler more than a few times — multiple resets can conceal an underlying persistent fault and waste time.

Information to collect for a professional visit

- Exact fault code(s) and any sequence of events (when it first appeared, what you changed just before it appeared).

- Whether Comfort/preheat was on or off and the current target flow temperature setting.

- Any other symptoms: loss of hot water, loss of heating, loud noises, low pressure, or other fault codes.

- A photo of the display and front of the boiler can save time for the engineer.

When to call a professional

- Call a Gas Safe registered engineer or Vaillant service if D.7 returns after you have checked and reset settings, if the boiler is not supplying heating or hot water, or if other fault codes appear alongside D.7.

- An engineer will check controller communication, verify flow/return sensors and wiring, test the warm-start logic in the PCB, and replace faulty sensors or repair wiring if required. These diagnostics and any internal repairs must be done by a qualified technician.

Do not attempt

- Do not open the boiler casing, work on gas/electrical components, or replace sensors yourself. These are specialist tasks that require a qualified engineer.

If you need help finding a qualified engineer, contact Vaillant customer service or use a local Gas Safe registered installer. Provide the engineer with the D.7 code, your notes and any photos you took to speed diagnosis.