BMW 530d: Runs on 5 Cylinders Only

My 530D, manufactured in 1999, with 148,000 km, runs great and consumes 6.5 liters per 100 km, but sometimes, when the engine is warm, it has the habit of only running on 5 cylinders at idle.
Diagnosis at BMW in Munich-Ottobrunn. Control unit is okay, injectors are okay, turbocharger is defective (cannot be down-regulated at idle).
A service technician, contacted directly by the factory, had examined the engine and identified a faulty injector.
Is there anyone here who has an opinion on this? How can injectors (that are externally sealed) be tested?
Thank you very much already

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