That's not necessarily true. Some faults will self-heal, meaning that a code is stored but the light on the dash doesn't come on. Certain overboost conditions trigger fail-safe or limp modes very quickly, underboost (as it's considered a safer condition than overboost) takes a long time to register as a fault. Actuator position faults (stripped gears, stuck mechanism etc) would be stored as a fault code. I would definitely start with hoses (boost leaks, etc) before going too deep into the expensive bits!