Error D.35

Safety precautions:

1. If you smell gas, leave the property immediately, call the gas emergency number and do not operate electrical switches or the boiler. 2. If no smell of gas, switch the boiler off at its mains isolator and at the boiler front panel before doing any inspection. Do not remove the boiler casing or attempt internal electrical or gas repairs. 3. Only perform non-invasive checks described below; any work on gas, internal wiring, sensors, pump, valves or PCB must be done by a Gas Safe engineer.

Initial homeowner checks (safe, non-invasive):

1. Note the exact error code (D.35) and any other codes or messages, and whether it appears immediately on restart or after running for a while. 2. Try one controlled reset using the boiler reset button (follow the manual). If the code returns immediately or after a short run, stop resetting and proceed with checks. 3. Check the boiler system pressure gauge when cold. Typical target is around 1.0–1.5 bar (consult your manual). If pressure is well below 1 bar, top up using the filling loop per the boiler manual instructions. After topping up, switch the boiler back on and observe behavior. 4. Look for visible leaks around the boiler, pipework and radiators. Any sign of a leak means do not run the boiler and call a professional. 5. Check condensate pipe and discharge for visible blockages or freezing (especially in cold weather). 6. Run the heating demand (turn thermostat up) and listen at the boiler for the pump running; feel the external pipework (careful—may be hot). If the pump appears not to run while demand is present, make a note for the engineer.

Further diagnostic steps you can do safely:

1. Bleed radiators to remove trapped air (start with top floor radiators). After bleeding, re-check and re-pressurise the system if pressure has dropped. 2. Record when the fault happens (on hot water, heating, immediately at start-up, after some minutes) and any odd symptoms (grinding pump noise, cold radiators, fast temperature rise, frequent pressure loss). This information speeds up a professional diagnosis. 3. Check for any obvious external damage to cables or connectors that are accessible without opening the boiler casing. Do not disconnect internal plugs or perform continuity tests unless you are qualified.

When to call a professional and what they will check:

1. If D.35 persists after the simple checks above, or if you were unable to correct low pressure, found leaks, or observed the pump not running, call a Gas Safe registered engineer. 2. Do not attempt internal repairs, sensor replacements, gas valve tests or PCB work yourself.

3. The engineer will: verify flow and return temperatures, measure NTC sensor resistances and wiring continuity, test and inspect the pump for flow and correct operation, check the diverter valve and heat exchanger for blockages or sticking, test the water pressure/flow sensor and expansion vessel, inspect the gas valve/safety circuits and ignition if relevant, and scan for PCB or wiring harness faults. They may purge or power-flush the system if flow problems or debris are found, and replace defective sensors, pump, valve or electronics as required.

Final notes and what to tell the engineer:

1. Provide the engineer with the boiler model, the exact code D.35, when it occurs, whether you tried a reset and what happened, current system pressure, and any evidence of leaks or unusual noises. 2. Avoid repeated resets—persistent safety shutdowns need a professional. 3. If the boiler was recently serviced or worked on prior to the fault appearing, mention that to the engineer as it can help trace wiring or sensor issues.