BoilerFaultCodesWorcester Bosch Fault 12 / 171 Fault Code
Also written as FAULT12171 — same fault.
The Worcester Bosch Fault 12 / 171 fault code means: your boiler has lost communication with its internal electronic controls or a safety switch has triggered to prevent overheating.. It appears on 1 Worcester Bosch model in our database.
Technical description: System configuration: BUS device missing. HLC has tripped or is faulty.
What causes the Worcester Bosch Fault 12 / 171 fault?
This fault occurs when the boiler's main control board loses its connection to vital internal components or when a safety sensor detects that the water inside is getting too hot. The most common reason is a tripped High Limit Stat, which acts like a safety fuse to shut the system down if it senses a risk of overheating. In some cases, it can also be caused by loose internal wiring or a communication error within the electronic control unit.
This fault is slightly more common in winter when the heating system is running for long periods, which can cause older components to overheat or reveal underlying circulation issues.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Worcester Bosch models that show the Fault 12 / 171 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch Fault 12 / 171 fault code information on this page is sourced and cross-checked against the official Worcester Bosch service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026