A thorough pre-tuning inspection is the foundation of a successful calibration. Tuning a vehicle with existing mechanical issues can mask problems, worsen existing faults, or prevent the calibration from delivering its intended results.
Essential checks include: engine oil level and condition, coolant system integrity (no leaks, correct level), boost system integrity (check for boost leaks using a smoke test), timing chain condition (especially N47, N20), active fault codes (scan and address before tuning), service history (is the vehicle up to date?), fuel system (correct fuel, no contamination), and turbo condition (no excessive shaft play or oil consumption).
For diesel engines, additionally check: DPF condition and soot loading, EGR valve function, injector balance rates, and glow plug operation.