Running a comprehensive diagnostic scan before tuning serves two critical purposes: it identifies any existing issues that must be resolved first, and it establishes a baseline that the calibration engineer can reference if questions arise after tuning.
A pre-tuning diagnostic should cover: all ECU modules (not just engine), live data readings at idle and under load, adaptation values, long-term fuel trims, injector correction values (diesel), and any pending or stored fault codes.
Store the diagnostic results securely — they serve as your "known good" reference point. If any fault codes appear after tuning, comparing against the pre-tuning scan helps distinguish between calibration-related and pre-existing issues.