Error D.0

Safety precautions:

1. If you smell gas, evacuate the property and call the gas emergency number immediately — do not operate electrical switches or the boiler. 2. Always isolate electrical power to the boiler before opening covers. 3. Work only within the safe visual checks described below; do not attempt gas, PCB or sealed-component repairs yourself. Call a Gas Safe engineer for any gas or internal-electrical work.

Initial homeowner checks (safe, no tools or only basic tools required):

1. Note exact symptoms and when D.0 appears (e.g., on CH demand, intermittently, after a reset). Record any other fault codes or F‑codes shown. 2. Check boiler and system pressure on the gauge. If pressure is below ~1.0 bar, repressurise using the filling loop according to the boiler manual, then re-check for the code. 3. Check that the programmer/room thermostat and any external controls are calling for heat: set the room thermostat higher than room temp and put the programmer into a CH on mode. 4. Make sure radiator TRVs are open and at least one radiator is hot to confirm there is a genuine heat demand. 5. Try a simple reset of the boiler (use the reset button as per manual) and see if D.0 clears and whether the boiler runs normally for a period. 6. Listen for the pump running and feel the flow and return pipes after a short run; if both pipes stay very similar temperature it can indicate poor flow or short cycling.

Basic diagnostic steps you can safely perform:

1. Bleed radiators to remove trapped air which can cause poor circulation and part-load behavior. 2. Check any visible external wiring/connectors (e.g., thermostat wires, external pump/valve connections) for obvious loose or damaged plugs — only inspect visually; do not disconnect or probe live circuits. 3. Confirm other gas appliances work to rule out a mains-gas issue. 4. If the code is present but the boiler will heat, note whether it short-cycles (boiler fires briefly then stops repeatedly) or runs but never gets into condensing mode — short cycles point to mismatch between boiler modulation and system load or to control/sensor faults.

When to call a professional (and what they will check):

1. If the D.0 persists after the simple checks above, or if you see associated F‑codes (sensor faults, flow/return NTC errors), contact a Gas Safe registered engineer. 2. A qualified engineer can access the full diagnostic menu safely, check and, if appropriate, adjust the d.00 range-rating setting or down-rate the boiler to match your heating load. They will also test flow and return NTC sensors, the wiring harness, pump operation, motorised valves, and the PCB for faults. 3. Engineers will use multimeters/ohmmeters and manufacturer procedures to confirm sensor resistances, replace faulty NTCs or harnesses, and reconfigure parameters only when safe and correct to do so.

Important notes and final advice:

1. Do not attempt to change installer-level diagnostic settings (d.xx) unless you are a trained engineer. Incorrect settings can make the boiler unsafe or damage components. 2. Many causes beneath this symptom (sensor short/interruption, pump failure, wiring harness or PCB issues) require specialist testing and Gas Safe competency. 3. If in any doubt, or if other alarms/faults appear, book a Gas Safe engineer to inspect the boiler and correct the configuration or replace faulty components.