Is it possible to read engine codes in a 95 Bonneville with no scan tool?

No, you can't directly read engine codes from a 1995 Pontiac Bonneville without a scan tool (OBD-II scanner). While some older vehicles had rudimentary methods of displaying codes through flashing lights on the dashboard, the 1995 Bonneville's system relies on a digital communication protocol that requires a scan tool to interpret the stored Diagnostic Trouble Codes (DTCs).