Based on the popularity of my LED Interior Lighting post, and by popular request in the comments section, I decided to write another guide for LED License plate bulbs on a B7 Audi A4, S4 and RS4. They definitely look awesome, and are a relatively cheap mod at ~$30 total. Best of all, installation takes merely a few minutes and all that is required is a small screwdriver in most cases.
The trouble with this mod is that the with some B7 models (any A4, S4 or RS4 from 2005.5-2008), the installation of these bulbs sometimes causes a Light Out error. Personally, I installed mine and never had any problems with a bulb out error message or flickering issues, and I’ve talked to countless others who have never had problems either, but there are more than a handful of examples of those who have posted on Audizine who did encounter problems.
There are many theories as to which models or trim packages throw the Bulb Out error and which don’t, but from everything I’ve read, it’s mostly just luck of the draw/random. Many believe that if you have the multi-color information display in your gauge cluster (the LCD screen between your speedometer and RPMs) you are more likely to have an bulb out warning message, although Bulb Out errors do occur for the non-color version (those with only red colored pixels only).
Regardless, here are my recommendations – IMO it’s worth trying your luck as you can always revert back to the OEM halogen bulbs if you get a bulb out message, learn to ignore the bulb out message in your gauge cluster, or install an extra resistor to avoid this (more on this below). A few others have reported flickering issues with LED license plate bulbs, which can also be fixed using resistors.
For bulbs, look for a 39mm Festoon bulb that has a BUILT IN RESISTOR, or plan on purchasing your own resistor or load balancer to eliminate any issues. The built-in resistor is critical, as it will greatly reduce the chances of a bulb out message. The two options I’ve seen with the best reviews and success rates are
Option 1 - Buy “Error Free” LED License plate bulbs. Note they are not always error free for B6/B7 Audis due to our electrical systems, although they are less likely to cause errors than the typical 39mm Festoon bulb:
- 42 Draft Designs – this is what I’ve been using for ~2 years without problems. They say on their site not to use these bulbs for a B7 A4, but that’s just so they’re not liable to accept returns in the case you get a bulb out error – essentially, you must assume all risk they might not work or work but product codes.
- TunerDomes – their bulbs also include built-in resistors to help prevent error codes. I haven’t tried these personally but know a few people who run them. Like the 42 Draft Designs bulbs, a lot of people are running these successfully without any codes or errors, but a few here and there have reported error codes or flickering – again, it’s really kind of luck of the draw….(read a review here on Audizine from user Quattro)
Bulb Installation: lift up your trunk lid, remove your license plate for a little extra wiggle room, then remove the two screws holding the light cover in. You’ll probably need a specialty screwdriver like the ones used on eyeglasses as its a very small screw. Then pull out the old bulbs and snap in the new bulbs. Be sure to test it before re-installing the light covers, just to make sure the bulbs are making contact (you may need to bend the contact points of the base) as well as to see if you get any bulb out errors first.
Resistor Installation: if you need to install resistors, you’ll need to remove the liner of your trunk to access the wires. There are two screws that come out using a Phillips screwdriver, pictured below. One is below the trunk release handle and the second one is in a similar location on the opposite side of the trunk. Once the screws are removed, then there are four clips on the top edge and four clips on the bottom edge which will release when you firmly tug near each clip.
Once the trunk liner is pushed to the side, install the resistor. You’ll want to tap the brown wire and the purplish-gray wires that are furthest to the left of the harness. I found easiest to remove the harness to install the resistor first.
I find it works best to strip part of the brown wire and gray/purple wire first so the resistor makes good contact, as I find the red-plastic splicers don’t always cut all the way through. It doesn’t matter which wire you attach to which, just make sure the resistor bridges the brown wire and gray/purple one. Warning: the resistor gets VERY hot VERY quickly so don’t touch it once you’ve turned the lights on. I mounted the resistor using 3M double sided tape and then plugged back in the harness and turned the car on to test that the lights still turn on and no error warnings – success! If you still get a warning, go back and check our connections and make sure the exposed wires are lined up correctly within the red plastic splicer.
Disclaimer: I’m not responsible if you buy any bulbs and they don’t work, nor any damage you may cause to your car while working on it. Any modifications you perform on your vehicle are done at your own risk. That being said, there isn’t too much of a risk of causing any damage here, except you may be out the cost of LED bulbs in the event they flicker or cause a bulb out message.