Error 235

Safety precautions:

1) If you are not competent with gas appliances or mains electricity, stop here and call a Gas Safe engineer. Working on the boiler’s internals can be dangerous. 2) Before opening the boiler isolate electrical supply at the mains and isolate gas at the gas service cock if instructed by your engineer. Allow the appliance to cool. 3) Do not run the boiler if you suspect a gas leak; evacuate and call the gas emergency number.

Initial checks a homeowner can do (no tools or limited access):

1) Simple reset: try a reset on the boiler by pressing and holding the reset button for 3 seconds (or follow the GB162 reset procedure in the user guide). Wait and see if the error clears and the boiler attempts to start. 2) Power-cycle: switch the boiler off at the isolator or fuse for 30 seconds, then switch back on and check the display. 3) Check recent work: think whether the boiler was recently serviced or a part replaced (code plug / HCM / control board). If yes, tell the engineer — a wrong/new part is the common cause. 4) Check the display info menu: on some models press the spanner/return button to view the cause code and any extra info; note down the fault + cause to give to the engineer.

If you can access the boiler internals and are comfortable with safe isolation (only do this if competent):

1) Isolate electrical supply at the mains before removing any panels. 2) Remove the front panel according to the manual and check that the small KIM/code plug or HCM module is present and seated correctly in its connector on the burner control unit. A loose or unseated module can trigger errors. Carefully reseat — do not force — ensuring the connector is aligned. 3) Refit the panel, restore power and try a reset/power-cycle. If the error clears, monitor operation and book a service to identify why it became loose.

If reseating or resetting does not help:

1) Do not attempt to modify or replace the code module or control board yourself unless you are a qualified engineer. The likely fixes are: replacing the KIM with the correct/compatible version for that boiler variant, or updating/replacing the burner control PCB to a version that accepts the newer KIM. Either option requires the correct part numbers and manufacturer guidance. 2) If there was a recent part swap, restore the original module (if you have it) and try again — but only if safe and you are competent to do so.

What information to give the engineer and next steps:

1) When you call a Gas Safe engineer or Worcester Bosch support, report: boiler model (GB162), error code 235, any cause-code shown in the info menu, whether any parts were recently replaced or the boiler serviced, and whether you tried reset/reseating. 2) The engineer will check compatibility of the KIM/HCM and control board, may read service logs, check connectors, and will replace either the KIM or the burner control unit with the correct matching parts or install a firmware update if available. 3) The engineer must carry out any part replacements, wiring or software updates. This is not a DIY repair for non-qualified people.

Final note: do not attempt gas valve, burner, control board or wiring repairs yourself. Error 235 is fundamentally a parts/compatibility issue; simple homeowner actions are limited to reset, power-cycle and checking that the identification module is present and properly seated. If those do not clear the code, call a Gas Safe registered engineer or Worcester Bosch support to carry out the correct diagnostic and replacement with compatible parts.