Error D.26

Safety first

1) If you smell gas, leave the building immediately and call the gas emergency number — do not touch the boiler. 2) Do not attempt internal repairs or gas/electrical work unless you are a qualified engineer. 3) Before any visual checks on terminals or modules, switch the boiler off at the mains and isolate power. If you are not comfortable doing this, stop and call a Gas Safe engineer.

Initial homeowner checks (safe, non-invasive)

1) Note the exact error code and any other codes or LED indicators shown. 2) Try a simple reset: press and hold the reset button (refer to your manual for the correct button) until the boiler attempts to restart. Watch the display — if the code clears and the boiler works normally, monitor for recurrence. 3) Check mains power and gas supply: ensure the boiler has mains power (other appliances on same circuit) and other gas appliances are working. 4) Check boiler water pressure on the gauge — if pressure is very low (e.g. <1 bar) the boiler can behave oddly; top up if safe and familiar with the filling loop instructions in your manual. 5) Check external controls: timers, thermostats, room stats or programmer that control heating/domestic hot water — ensure they are calling correctly and their batteries (if wireless) are OK.

If D.26 persists after reset or affects heating/hot water:

1) Look for any external relay modules or accessories installed on the system (e.g. cylinder modules, external pump relays, accessory control boxes). Note makes/model and whether any recent work was done on them. 2) If an accessory module is fitted externally and is accessible, with the boiler isolated at the mains you may visually inspect it for obvious damage, burnt connectors, loose wiring or tripped fuses. Do not open or probe circuits while live. 3) If the accessory or relay has an easily replaceable plug-in fuse (external low-voltage fuse) you can replace that fuse only if the fuse is externally accessible and you are confident — otherwise leave to the engineer. 4) If you can safely and easily disconnect the external accessory from the boiler (plug/socket type connector, not hardwired) try disconnecting it, then restore power and see if the fault clears — this can indicate the accessory is causing the fault. Do not short or bridge terminals.

Further diagnostic guidance (for engineer or if you are qualified)

1) Engineer should check continuity and correct voltage on the relay/control terminals and the wiring harness for shorts to earth or to other conductors. 2) Check any low-voltage fuses on the control board and any external accessory fuses. 3) Test the external relay coil and contacts and swap or replace the external relay/module if faulty. 4) If wiring and accessory are OK, the engineer should inspect the boiler PCB and associated drivers that monitor the relay input/output for faults and replace the PCB or part if necessary.

When to call a professional and what to tell them

1) Call a Gas Safe registered engineer if the code repeats, the boiler locks out, the boiler will not provide heating or hot water, or if you’re not confident performing the safe checks above. 2) When you call, provide: boiler model and serial number, exact error code (D.26), whether the code cleared with a reset, frequency/timing of the fault, any external accessories connected, and any recent work done on the boiler or controls. 3) Ask the engineer to check the external relay/module, wiring to the boiler, low-voltage fuses and the boiler’s PCB.

Important reminders

1) Do not attempt to replace or repair gas components, the PCB, or perform live electrical fault-finding unless you are qualified. 2) Do not bypass safety circuits or short terminals to force the boiler to run. 3) Repeated D.26 faults should be investigated promptly to prevent loss of function and to avoid potential secondary damage to pumps or valves.