July 1: So it begins. Don't worry, this is not in the correct legal jurisdiction!
DTC P1621: Notes
WARNING: This page is about a different car, the 2005 Mazda MX-5 Miata. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: DTC P1621 [BP, BP With TC] Detection Condition And Possible Cause