Overview
D.0 (often shown as d.00 in the Vaillant diagnostic menu) is a diagnostic code related to heating part-load behaviour — essentially the boiler’s internal settings or operating conditions that control how the burner and pump run against the heating demand. On Vaillant boilers this diagnostic item can show the boiler’s heating range rating or part-load setting and is used to match the boiler output to the house heat load (down‑rating or range rating). If the setting, system flow or control behaviour is not matched to the radiators and pipework it can cause very short firing/condensing cycles, poor system warm-up and reduced efficiency. This is not usually an immediate safety shutdown like an F‑code for flame or pressure faults, but it can cause comfort issues (radiators take too long to warm or don’t reach temperature) and higher fuel use. The underlying causes range from a simple mismatch of installer settings to problems with flow/return sensors, low water pressure, inadequate pump flow or airlocks in the system. Basic checks can be done by a homeowner but diagnosis and any changes to diagnostic settings, sensor replacement or electronic repairs should be completed by a Gas Safe registered engineer. Changing 'd' settings or working on internals without qualifications can create unsafe conditions or invalidate warranties.
Possible Cause: Heating part load error
Troubleshooting Steps
Safety first
- If the boiler is showing other alarm/fault codes or has gone to lockout, switch the boiler to the OFF position and do not attempt internal repairs. Contact a Gas Safe engineer.
- Do not remove the boiler casing or touch internal components. Working on gas appliances or electrical components is dangerous and must be done by a qualified engineer.
- If you suspect a leak or smell gas, evacuate the property and call the gas emergency number immediately.
Initial homeowner checks (safe, simple checks you can do)
1. Check the boiler display and note the full code and any F‑codes displayed alongside D.0. Take a photo of the display and write down the time.
2. Confirm the heating demand settings: make sure the central heating thermostat and programmer/timers are calling for heat and room stat set above room temperature.
3. Check system water pressure on the boiler gauge. Typical pressure when cold should be about 1–1.5 bar. If pressure is significantly low (below about 0.7–0.8 bar) top up using the filling loop according to your boiler manual. Only use the correct filling method for your model.
4. Bleed radiators to remove air. Airlocks and trapped air reduce flow and can cause large flow/return temperature differences that trigger diagnostics.
5. Ensure any isolation valves feeding the boiler and radiators (lockshield and valve under the boiler) are open. Check for obvious leaks or visible damage.
6. Listen at the boiler for the pump running when heating is on. You should hear a faint hum or feel vibration; total silence can mean a failed pump or power to the pump is interrupted.
7. Try a simple reset: switch the boiler off at the programmer and the boiler power (or use the reset function on the boiler), wait one minute, and restart. Note whether the D.0 message persists.
Viewing diagnostic information (do not change settings)
- Many Vaillant displays allow you to view diagnostic values: press and hold the information (i) button and the + button together to enter the diagnostic menu. Scroll to the D.0 (or d.00) entry and simply record what the display shows. Do NOT change any values — installer changes affect safety and efficiency and should only be made by a competent engineer.
- Record any flow/return temperatures or readings shown and whether the boiler was calling for heat while you took the reading.
If the problem persists after the homeowner checks
- If topping up pressure, bleeding rads, ensuring valves are open and a reset do not clear the issue, do not attempt further internal checks. Problems that commonly sit behind D.0 or related heating performance diagnostics include: incorrect range rating/down‑rating set in the installer menu, faulty flow or return NTC temperature sensors, wiring/connectors to sensors or PCB faults, pump performance issues, and blockages/airlocks or debris in the heat exchanger or system.
What to tell the engineer and what they will likely do
- When you call a Gas Safe registered engineer give them: boiler model, serial number, the exact displayed code (D.0 or d.00), any accompanying F‑codes, what you have already checked (pressure, bled rads, reset), and photos of the display.
- A qualified engineer will safely open the boiler, check installer diagnostic settings (range rating/down‑rating), measure NTC sensor resistances and flow/return temperatures, test pump current/operation, inspect wiring and the PCB connections, remove airlocks, and check for blockages or debris in the system. They may adjust the range rating only if appropriate for the house load, repair/replace faulty NTCs, harnesses, pump or PCB as required.
Reminder and final advice
- Do not change installer settings or carry out electrical/gas work yourself. If you are uncomfortable with any of the above or the D.0 message persists after basic checks, contact a Gas Safe registered engineer to diagnose and repair. Arrange a full service if the boiler has not been serviced recently, as many of these issues are uncovered or prevented during routine servicing.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Gas Boiler.