Known-issue checklist by chassis & engine
- N20 / N26 4-cyl turbo (228i, 320i, 428i, 528i, X1, X3 ~2012–2018): timing chain guide failure — we listen at cold start and review service history
- N54 twin-turbo (135i, 335i, 535i ~2007–2013): HPFP, wastegate rattle, injector index codes, oil filter housing leaks
- N55 single-turbo (135i, 335i, 535i, X3, X5 ~2010–2018): VANOS solenoids, water pump, oil filter housing gasket, valve cover gasket
- B58 (340i, 440i, 540i, X3 M40i, Z4 M40i, Toyota Supra): generally robust — we check for oil filter housing gasket weep and charge pipe condition
- S55 (M2, M3, M4 F-chassis): rod bearing wear interval, charge pipe upgrade, cooling system condition
- N63 / S63 V8 (550i, 750i, X5/X6 5.0i): oil consumption, valve stem seals, timing chain — expensive to fix, critical to inspect
- 8HP ZF transmission: fluid condition (BMW calls it 'lifetime', it isn't), mechatronic sleeve leak
- iDrive version, CIC/NBT/NBT-Evo/ID7 — affects CarPlay availability and update path

