9U 233 on a Worcester Bosch boiler means: heat control module (HCM) code plug faulty or disconnected. The boiler's control electronics have detected an internal fault.
This code applies to Worcester Bosch Greenstar Si Compact, Greenstar i and CDi Compact. Codes can mean different things on different models, so check the display against your boiler's user manual.
What you can safely check
- Turn the boiler off at the switch or fused spur, wait 30 seconds, and turn it back on.
- Press reset once, following the instructions in your user manual.
When to call an engineer
If the code comes back, call a Gas Safe engineer. A control board replacement can be one of the more expensive repairs, so on an older boiler it is worth getting a replacement price too.
Never remove the boiler casing yourself — everything behind it must be handled by a Gas Safe registered engineer. If you smell gas, open windows, turn the gas off at the meter, and call the National Gas Emergency number on 0800 111 999.
Repair it, or replace it?
If your boiler is over 10 years old, compare a repair against a new combi before you pay for parts.
Applies to: Worcester Bosch Greenstar Si Compact, Greenstar i and CDi Compact
Meaning checked against: HomeServe — Worcester Bosch boiler faults
Last reviewed November 2026. Always follow your boiler's own user manual.