Your boiler is currently set to run at its maximum heating capacity, which is typical during a start-up or testing cycle rather than a permanent fault.

Service Function: Max. heating output

What causes this fault?

This code appears because a service engineer has manually activated the 'chimney sweep' mode using the boiler's internal settings. It is not actually a fault, but rather a deliberate setting that forces the burner to run at full power so that gas pressure and emissions can be accurately tested. If you see this code and an engineer is not present, the service switch may have been left in the wrong position or bumped accidentally.