Error D.90

Safety precautions:

1) If you are not comfortable working around electrical equipment do not open the boiler casing. 2) Before touching any wiring or internal connectors, switch the boiler off at the mains isolator and, if present, the external gas isolation cock. 3) Do not attempt to alter gas or combustion components; leave that to a Gas Safe engineer. 4) Take note of the exact code (D.90) and any other codes or display messages and the conditions when they occurred (heating on/off, hot water demand, time of day).

Initial checks a homeowner can do (no tools or minimal effort):

1) Reset the boiler once: press and hold the fault-clear/reset button for one second (follow the boiler display instructions). Wait and observe whether the code returns. Avoid repeated resets—if the fault returns immediately, further intervention is needed. 2) Check the external regulator/room thermostat: if it is battery powered, replace the batteries. Make sure the regulator is powered on and set to call for heat (set temperature above room temperature to create a demand). 3) If the regulator is wireless, ensure the gateway or repeater (if fitted) is powered and that there are no obvious obstructions or long distances causing loss of signal. 4) Check the programmer/timer: ensure the boiler is not being prevented from running by a timer or holiday mode. 5) Note if any other faults appear on the display or if heating/hot water behavior is abnormal.

More detailed checks you can do if you are competent with basic safe checks (DO NOT work on live circuits or inside the boiler with power on):

1) With the boiler powered off at the mains, open the front panel only if you are comfortable doing so and the manufacturer instructions allow. Visually inspect the regulator wiring/connector at the terminal block for loose plugs, disconnected wires or corrosion. Reseat any multi-pin plugs gently if they appear loose. 2) If the regulator is wired, check that the wires between the regulator and the boiler are secure at both ends (regulator and boiler terminals). Do not cut, strip or splice wiring yourself if you lack experience. 3) If you have access to the user/operator menu, use the boiler Live Monitor/Status screens to check the digital regulator status messages or any additional diagnostic numbers which may give context to D.90. Record any messages or values to tell the engineer. 4) Check system basics: confirm boiler system pressure is within the normal range on the pressure gauge and that there are no other obvious system faults that might confuse diagnostics.

When to call a professional and what to expect:

1) If D.90 returns after a single reset, if you cannot locate or fix a simple regulator battery/setting/wiring issue, or if you find loose or damaged wiring, stop and call a Gas Safe registered engineer. 2) The engineer will have tools and test equipment to measure communications on the eBUS or thermostat wiring, test resistances and continuity, check the regulator device itself and the boiler PCB, and replace faulty components (regulator, wiring harness, connectors or PCB) as required. 3) Tell the engineer you have a persistent D.90 code and provide any other codes or Live Monitor data you recorded, plus the make/model of the regulator if it is separate from the boiler. 4) Do not repeatedly reset the boiler to force operation; this can mask the underlying issue and may cause further faults.

Final note: D.90 commonly turns out to be a problem with the external regulator, its batteries or its wiring/communication link—often a simple fix—but because the diagnosis can require electrical and PCB-level testing, a Gas Safe engineer should carry out any internal electrical repairs or component replacements.