Sometimes, the B173D is a phantom code . The Battery Management System (integrated into the negative battery terminal sensor) loses synchronization with the relay’s actual state. This happens after a battery replacement without proper coding.
While a brake switch sounds like a minor component, modern Peugeots use this signal for: b173d peugeot work
Reconnect, wait another , then turn on the side lights and ignition without starting for a minute to allow the BSI to re-initialize. Sometimes, the B173D is a phantom code