:wink2:Slightly longer answer... I believe this is related to a certain vehicle model being able to support either halogen or LED bulbs, depending on chosen factory options. Thus, for such models VW designs the electrical circuit to support both situations, meaning that only a software configuration is needed in production (i.e. VAS ) to detect both halogen and LED bulbs correctly depending on chosen factory option.
For example, some of the newer sedans (don't remember the model now) can be ordered with either halogen or LED license plate lights, and the electrical control module is then adapted via software configuration to apply the correct detection "algorithm".
Thus as both the T1 and T2 were only factory equipped with halogen bulbs throughout (well, except the side blinkers on mirrors and center brake light), the faulty bulb detection circuit detects only halogen bulbs correctly.
So yes, nooby's short answer is correct. And in addition, halogen and LED bulbs have different radiation patterns, and once you fit a LED bulb into a reflector housing designed for a halogen bulb, you will change the reflection pattern of your light quite drastically...
