30FF

N54 cars have been setting 30FF since they reached dealers in 2006. The engine has two turbos, and one failing turbo is enough to set it. The first measurement is boost itself: intake manifold pressure against ambient, at idle and under load. A manifold that pulls a vacuum under load is getting no boost.

The wastegates are vacuum-controlled, so the next checks follow the vacuum: the supply from the vacuum pump (around 29 inHg), the storage tanks, the fabric-covered hoses — which crack at flex points and bends — the two electric control solenoids, which a scan tool can actuate, and the wastegate actuator diaphragms. A smoke test of each section finds leaks without taking anything apart.

If all of that holds, the problem is inside the turbos. In the case The bimmer pub documented — a 2009 535xi at about 138,000 miles — the vacuum side tested good, and the wastegate valves on both turbos had worn loose while the turbines themselves spun freely with no wear.

Turbochargers get quoted on sight of this code. Sometimes the turbos are the answer — but a cracked vacuum hose, a dead control solenoid or a worn actuator diaphragm sets the same code, and the vacuum side is tested before anything is unbolted.