The legal bottom line for a Z3 (1996–2002)
Because every BMW Z3 was first used after 1 April 1986, converting its original halogen headlamp units to use LED (or HID/xenon) light sources is an MOT fail for “light source and lamp not compatible.”Swapping the internals (e.g., opening the housing and fitting bi‑LED projectors inside the factory lamp) voids the lamp’s type‑approval, so even if the beam looks good, it’s not road‑legal. UK guidance makes clear that headlamps are approved as a complete unit (outer lens + reflector + light source), not as mix‑and‑match parts. If you want LED or HID legally, the car must use a complete, E‑marked headlamp assembly designed and approved for that light source. For HID in particular, DfT expects compliance with UN ECE R98 (component) and R48 (installation), which brings in headlamp cleaning and automatic levelling where applicable