Most people shopping for a high-mileage N54 335i have no idea they're one HPFP fault code away from a $2,500 repair bill before they even get to enjoy the car. Here's what to check before you hand over cash: 1. Wastegate rattle — rev the engine cold and listen for a metallic chattering around 1,500-3,000 RPM. If both turbos are rattling, you're looking at a twin turbo replacement that'll run $3k-$5k installed. This is the single most expensive overlooked issue on these cars. 2. HPFP codes — pull codes with an OBD2 scanner and look for 2FBF and 29DC. The high-pressure fuel pump is the N54's most notorious weak point and BMW extended the warranty on it for a reason. If that warranty window is closed on a high-mileage example, the replacement is on your dime. 3. Injector drift — request an injector test or check live data for flow rate deviations. Index 1-7 injectors have varying failure rates, and when they start misfiring under boost you're chasing phantom codes for weeks. A full se...