Error D.58

Safety precautions:

1) Do not open the boiler casing or attempt internal electrical, gas or component repairs unless you are Gas Safe registered. Isolate the boiler only if instructed by a qualified engineer or in an emergency. 2) If you smell gas, leave the property immediately, call the gas emergency number and do not operate electrical switches or the boiler. 3) Avoid repeatedly resetting the boiler—only reset after performing the checks below or when advised by a professional.

Initial checks a homeowner can do:

1) Note exactly when D.58 appears (sunny weather, hot water demand, after a period of sun/lack of sun). If you have a solar thermal system, check whether the water in the hot water cylinder or at a hot tap is already warm — this supports that solar reheating is active. 2) Look at the boiler display Live Monitor (if present) or messages to confirm any accompanying indications (e.g. DHW active, other fault codes). 3) Check the boiler’s main controls: ensure the boiler is powered on, the gas isolator is open, and the heating/DHW settings are correct. 4) Check boiler pressure at the display; low system pressure will cause other issues but is not normally the cause of D.58.

Basic resets and non-invasive steps:

1) If the message is transient and you have confirmed solar heating is plausible, you can wait — the boiler should resume normal operation when the solar input falls. 2) To clear a transient message, press and hold the boiler reset button for up to 10 seconds as per the manual. If the code returns immediately or the boiler still won’t provide heat when expected, stop and arrange professional help. 3) Power-cycle the boiler (switch off at the isolator for 30 seconds then back on) only if you are comfortable doing so; this can clear transient diagnostics but do not rely on repeated power-cycling.

Visual inspection steps you may perform safely:

1) Locate any external solar controller and sensor wiring you can see (usually at the hot water cylinder or on the solar controller). Look for obvious signs of damage, loose connectors, or corroded terminals. Do not unplug or plug in connectors inside the boiler casing yourself. 2) Check the hot water cylinder thermostat or sensor pocket (if accessible) for signs of moisture or a loose wire at the external connection point. 3) Check any visible pipework and valves associated with the solar loop for obvious faults (closed valves, visible leaks) but do not attempt to alter pipework or wiring beyond closing a visible service valve in an emergency.

When to call a professional and what they will do:

1) Call a Gas Safe registered engineer if you do not have a solar thermal system but D.58 appears, if the boiler will not supply hot water or heating when it should, if you see other fault codes with D.58, or if visual checks show loose or damaged wiring that you cannot safely isolate. 2) The engineer will check the solar controller, cylinder thermostat/NTC sensor(s), the sensor wiring and plugs, and the boiler’s actoSTOR/PCB connections. They can measure sensor resistances and temperatures, test continuity of the harness, confirm configuration settings, and replace defective NTC sensors, cable harnesses or connectors if required. 3) If the D.58 is caused by incorrect configuration or a fault with the solar controller, the engineer will correct settings or repair/replace the external controller or associated wiring.

Important warnings:

1) Do not attempt to replace NTC sensors, access the PCB, or work on gas or flue components yourself. These tasks must be carried out by a Gas Safe registered engineer. 2) Record the exact error code, any other messages and the circumstances (weather, recent work, when the code appears) before the engineer attends — this speeds diagnosis. 3) If the boiler displays other error or safety shutdown codes, or if there is any risk to occupants, isolate the boiler and call a professional immediately.