The most common problems
LED bulbs have polarity. If they are not connected the correct way round, they won't light up. In fact it is entirely possible to design an LED bulb that works either way round; it's just slightly more expensive. As a result, we all put up with polarised bulbs. This is not an issue with a T10/501 bulb which is mechanically symmetrical, but a bulb with an end-cap either requires the wires to the holder to be reversed, or the bulb to be re-engineered with reversed polarity.

One of the features of LED bulbs is their low current consumption. However, this can be a problem for cars which have bulb failure systems, most of which will report an LED bulb as being faulty because it does not draw enough current. This includes use as indicator bulbs, where a conventional circuit will flash too quickly (or in some cases, not flash at all).

There is quite a high failure rate for white LED bulbs, and part of the problem is caused by poor design. This is often simply due to over-driving the LEDs to make the most light from the fewest components. However, most of these bulbs have two or more parallel circuits of two or three LEDs in series. They all share the same current-limiting resistor, which means that one series string dominates (taking more current), reducing the life of the most vulnerable LED in that string. Many T10/501 bulbs with four LEDs are quickly reduced to two dimmed LEDs, a dead one, and a very bright one.

Replacement LED bulbs are not always bright enough to be safe for brake lights, and colours are not always what would be expected.