Error D.90

Safety precautions:

1) If you smell gas or see signs of fire or burning, leave the property immediately and call the gas emergency number. Do not try to operate the boiler. 2) Do not remove the boiler casing or work on internal wiring unless you are a Gas Safe registered engineer. Internal sensors, wiring and the PCB involve live electrical parts and gas safety systems. 3) You may safely switch the boiler off at the external on/off switch or the fused spur and isolate the electrical supply for the boiler while you perform external checks. Do not isolate the gas supply unless advised by a professional.

Initial homeowner checks you can safely do:

1) Note the exact error code(s) and any accompanying F.xx codes displayed before calling an engineer. Photograph the display and any other fault history or LED indicators. 2) Try a single reset: press and hold the boiler’s reset/key fault-clear button for one second as per the manual. Wait and observe whether the code returns. Do not press reset repeatedly more than three times if the fault persists. 3) Check boiler pressure on the gauge. If pressure is very low (below ~0.6 bar) the boiler may lock out with other faults; top-up to normal range only using the external filling loop if you are familiar with it. 4) Check external wiring and accessory connections you can access without opening the boiler: room thermostat/controller batteries and connections, wired controllers, and any external cylinder controller (actoSTOR) plugs that are mounted externally. Ensure controllers are powered and connected. 5) Inspect visible condensate pipe for freezing/blockage (common winter issue) and ensure condensate is draining freely. 6) Note any recent work on the central heating, electrics or plumbing that could have disturbed sensor plugs or wiring.

Specific diagnostic steps and what the professional will do (so you know what to expect):

1) If the code is linked to sensor interruption (common): a Gas Safe engineer will check flow and return NTC thermistors, DHW outlet NTCs and cylinder NTCs (if present). They will measure resistance at ambient and heated temperatures and test continuity of wiring harnesses and connectors. 2) The engineer will inspect all plug connections on the PCB and any external electronics modules (actoSTOR), looking for unplugged, loose or corroded plugs and for mass shorts where a plug is earthing to the casing. 3) If the issue is a short or open circuit in a wiring loom, the engineer will trace and isolate the fault. Harness or cable replacement is often required if damage is found. 4) If a sensor (NTC) is out of specification, the sensor will be replaced. If connectors are damaged, the plug or short section of loom may be repaired or replaced. 5) If testing shows correct sensor resistances but the PCB reports communication faults (eBUS), the engineer will test bus wiring and may need to replace the PCB or external controller if the eBUS is faulty. 6) If additional faults are present (e.g. safety cut-out due to pump failure, blockage, or ignition/flame sensing faults) the engineer will investigate pump operation, filters/hydroblock, flue and ignition components as required.

When to call a professional now:

1) Call a Gas Safe registered engineer whenever D.90 persists after a single safe reset or if you are not comfortable performing the simple external checks above. 2) Call immediately if you smell gas, notice water leaks, burning smells, or the boiler will not reset and the fault is causing loss of heating or hot water.

What information to give the engineer:

1) Boiler model and serial number (from the plate on the boiler), exact displayed codes (D.90 and any F.xx codes), when the fault started and any events or work done recently. 2) Photos of the display and any accessible connectors you inspected. 3) Whether the boiler resets and for how long before the code returns.

Final note: Many D.90 occurrences are caused by sensor or wiring faults which require instrument testing and replacement parts. Repairs to sensors, wiring harnesses, PCBs or any internal parts must be carried out by a Gas Safe registered engineer. Do not attempt internal repairs yourself.