The brief
Mr Ben's Golf had the headlights and signal lights going haywire, an electrical fault that needs a systematic approach rather than a guess, because lighting can play up for several reasons, the bulbs, the wiring, a fuse, or a control module. The exterior lights on this car are run by a control module, the J519 onboard supply control unit, which manages the headlights, the indicators and a lot of the body electrics. When it develops a fault inside, the lights it controls misbehave, flicker, stay on, stay off, or behave randomly, even though the bulbs and wiring are fine. So the job is to work through the possible causes in order, rule out the wiring and the fuse box, confirm it's the module, and then replace it.
The diagnosis
A systematic scan and tests went through the probable sources: the wiring checked out, no fault there, and the fuse box checked out, nothing contributing to the module failure. With those cleared, the cause was the J519 module itself, which is why the headlights and signal lights were haywire. That's a J519 module replacement with coding to the car, rather than chasing wiring or fuses that weren't the problem.
The work
With the wiring and fuse box confirmed clear, the faulty J519 onboard supply control module was removed and a new genuine VW-spec module fitted, then coded and programmed to the car so it manages the lighting and the body electrics correctly. The fault codes were cleared and every light and function checked. A road test and a function check confirmed the headlights and signal lights working properly, all functions normal, no fault codes, and no warning lights.
The outcome
The headlights and signal lights working properly, all the body electrics normal, no fault codes, and no warning lights. Mr Ben got the Golf back with the lighting sorted. Working through the wiring and the fuse box before condemning the module meant the right part got replaced, and coding the new J519 in is what brings the lights and the body systems back online.
