Not every vehicle needs a full diagnostic workup before tuning — but many do. Knowing when a diagnostic is essential can save significant time, money and frustration.
Always diagnose first if: the vehicle has over 100,000 km, the service history is incomplete or unknown, there are any warning lights or intermittent faults, the vehicle was recently purchased (especially from a non-specialist), the engine makes unusual noises, there is visible oil leakage, or the vehicle has N47 or N20 engines (timing chain concerns).
A diagnostic may be optional if: the vehicle is under 50,000 km with full service history, it's been recently inspected by a BMW specialist, there are no symptoms or warning lights, and the owner has documented the vehicle's condition.
When in doubt, diagnose. The cost of a pre-tuning diagnostic is trivial compared to the cost of tuning a vehicle with an underlying issue.