Error D.17

Safety precautions:

1. If you smell gas, leave the property immediately and call the gas emergency number — do not try to investigate the boiler. If there are signs of water leaks or electrical burning, switch off the boiler and isolate power at the fused spur if it is safe to do so.

2. Do not open the boiler casing or touch internal components. Repairs to sensors, wiring, the PCB or gas components must be carried out by a Gas Safe registered engineer.

Initial homeowner checks (safe, simple steps):

1. Record the exact error code(s) shown (D.17 and any other codes such as F.91). Take a photo of the display for the engineer.

2. Try a controlled reset: press and hold the reset button as per the manual (usually up to 10 seconds). Note if the boiler attempts to restart and whether the error returns immediately.

3. Check boiler pressure on the front display. If pressure is very low (<1.0 bar) top up per the manual only if you know how. Low pressure can cause other faults but usually will not create a sensor short circuit code.

4. Visually inspect the external plumbing and the area around the boiler for obvious water leaks, wet connectors or obvious burnt/corroded plugs visible without removing the cover. Do not open the boiler cover.

5. If your system has a separate hot-water cylinder or actoSTOR unit, note if any domestic hot water functions are affected and record any related error codes—this information helps the engineer.

6. If heating is required and the boiler is locked out, use alternative safe heating while awaiting repair; do not repeatedly reset the boiler if the code returns.

Diagnostic and fix steps a Gas Safe engineer will perform (do not attempt unless qualified):

1. Confirm and reproduce the fault using service menus and read any stored fault history (including any associated F‑codes).

2. Visually and electrically inspect the sensor wiring harness, plugs and connectors for corrosion, damage, water ingress or loose pins. Re-seat connectors and clean corrosion where appropriate.

3. Measure the thermistor (NTC) resistances at ambient temperature. Typical reference: heating return/flow NTC should read around 12 kΩ at ~20 °C (manufacturer-specific values should be checked). Test for short (very low resistance) or open circuit (infinite resistance) and for implausible readings compared with actual temperature.

4. Check continuity of the cable harness and insulation to rule out shorts between cores or to earth. Check the connection at the main PCB and any intermediate modules (e.g. actoSTOR electronics plug connections).

5. If readings or wiring are faulty, replace the defective NTC sensor(s) and/or the cable/harness and any damaged plugs. Re-check sensor orientation and installation (flow vs return must be fitted to correct pipe).

6. If sensors and wiring check out, test the PCB inputs for correct sensor voltage/ADC signals; replace or repair the PCB if it is not reading correctly.

7. If there are indications of overheating or flue issues (e.g. STB triggered, soot), inspect and clean the flue and test the flue temperature limiter; replace STB if faulty.

8. After any repair, run the boiler through heating and hot-water demands, confirm correct flow/return temperature readings, verify control and safety operation and clear stored faults. Ensure no other fault codes persist.

Important final notes:

- Provide the Gas Safe engineer with the exact D.17 code and any other codes you recorded and photos of the display; this speeds diagnosis.

- Do not attempt to replace sensors, harnesses or PCB parts yourself unless you are a qualified Gas Safe engineer. Working on gas appliances and their electronics is dangerous and illegal for unqualified persons.

- If the boiler repeatedly reports this or other sensor faults after repair, have the engineer perform a full verification of the wiring harness routing, sensor placement and PCB functionality to prevent recurring failures.