Fault 12 Code 171 — System configuration, HLC tripped or ISM1 no longer detected

What does Fault 12 171 mean?

The Worcester Bosch Greenstar CDi Boiler displays Fault 12 with code 171, indicating that either the HLC (Hot Water Load Control) has tripped, or ISM1 is no longer detected despite being previously configured.

• HLC (High Limit Cut-out) has tripped or is faulty
• ISM1 connection fault or module failure
• Active thermal disinfection causing system behavior

Click steps to track your progress.

1. 1

   Check the HLC for its status; if tripped, reset it as per instructions.

2. 2

   Verify the connection to ISM1 for any issues.

3. 3

   Check solar parameter T2 and confirm if thermal disinfection is currently active, as this can affect system readings. Contact a qualified heating engineer if the fault persists.