Error 9U 233

Safety first:

1) If you are unsure at any point, stop and call a Gas Safe engineer. Work inside a boiler involves live electricity and gas components. Do not attempt to replace gas or sealed components yourself.

2) Before any checks, switch off the boiler at the electrical isolator and at the fuse spur if you will be removing covers. If you only perform external checks (reset, visual inspection of external wiring), isolate only if comfortable doing so; otherwise leave isolation to the engineer.

3) If the boiler is under warranty or service contract, contact Worcester Bosch or your installer before opening anything — unsupervised access can void warranties.

Initial homeowner checks (safe, low-risk):

1) Note the full fault: write down the exact display (9U 233) and any additional flashing cause codes. Take a photo of the display and any indicator LEDs for the engineer.

2) Try a controlled reset: press and hold the reset or tick (✔) button for ~3–8 seconds until the display reboots. Wait and see if the boiler starts normally. A one-off firmware glitch can sometimes clear with a reset.

3) Power-cycle: turn the boiler off at the isolator for at least 1 minute, then turn it back on and re-check the display.

4) Check external supplies: confirm the boiler has mains power (other household sockets working), the gas supply is on and the meter hasn’t tripped, and incoming water pressure is in the normal range. These won’t fix a 9U 233 but rule out simple supply issues.

5) Look for obvious external signs of damage or water ingress at the boiler case, or loose external wiring or terminal covers. Do not open the main case for internal inspection unless qualified.

If the fault persists — limited diagnostic things an experienced homeowner can do with caution:

1) If you are technically competent and only if the boiler is isolated from mains power at the isolator, you may visually check that external connectors to any user-accessible modules and the thermostat wiring are seated. Do not remove internal covers or touch internal PCBs unless you are trained; many manufacturers advise only qualified engineers should access the internals.

2) Do not try to 'reprogram' or replace the code plug yourself. The code plug contains configuration tied to your boiler model; incorrect parts or handling can prevent the boiler working or cause incompatibility.

3) Do not attempt to solder or manipulate wiring connected to gas valves, ignition leads, or the PCB.

What a professional engineer will do (so you know what to expect):

1) A Gas Safe engineer will run a full diagnostics readout, check the cause code memory, and verify the HCM/code plug presence and integrity.

2) They may reseat the code plug and HCM connectors, check wiring continuity and connector pins for corrosion/damage, and test the control box communication lines.

3) If the code plug or HCM is faulty, the engineer will source and fit the correct replacement part and load any required configuration. They will also check for secondary damage (water ingress, PCB faults, blown fuses) and test the boiler through a restart and run cycle.

4) The engineer will reset the fault memory and confirm safe operation across heating and hot water functions.

Final notes and when to call a professional:

1) If a reset or power-cycle does not clear 9U 233, call a Gas Safe registered engineer or Worcester Bosch support. This fault generally requires specialist diagnosis and likely part replacement.

2) Provide the engineer with the exact model, serial number, and the fault code (9U 233) and any photos you took to speed diagnosis.

3) Do not delay if the boiler is cold and occupants are vulnerable — contact an engineer quickly. If you smell gas at any time, turn off the gas supply, ventilate the area, vacate the property and contact your gas emergency number immediately.

Remember: safe reset and external checks are fine, but internal electrical and gas repairs and component replacements must be carried out by a qualified engineer.