Hello,

I'm looking for some help. After taking my 2009 997.2 C4S Cabrio, to get it's annual state inspection, I found that the license plate lights were out.

I had previously changes these to LEDs, but my suspicion is that one of them has shorted something.

Here is what I know:

- Changing bulbs back to standard fuse style, and still no function.
- Verified that contacts are making good connection into license plate light receptacle/housing (black part).
- Verified each housing/receptacle has a good connection to the harness.
- removed rear bumper and verified that I am not getting any voltage to the harness located behind the bumper.
- Both tail lights appear to be working normally. (One thing to ask - does the vehicle have two fog lights or one ? Only driver side fog illuminates. Not sure if this was normal)

Here's what I'm missing: I don't have the fuse box layout document. I did go through EVERY fuse and none are blown.

I'm really at a loss for what to look for next. If I had shorted out a license plate light, how would I fix it ?


Thanks very much for any help !

 

afaik there isn't a simple 12v + fuse system, instead there is a control unit that controls power to the lights. So if resistance is to low or to high, it will not deliver any power.

 

On the rear fog....there is only 1, so no issue there.

On the license plate lights, how long did you wait after reverting to oem? For whatever reason, if the Led's caused an issue, it can take a bit for even the oems to start working again. Put them in and leave lights on for a bit...then check, and they will likely reset.

Good luck,
DC

 

Thanks for the feedback ! Time to stick the bumper back on, and I'll let the OEM bulbs stew for a while before checking...

Would never have guessed that a simple circuit like a license plate light that uses $1-2 in bulb, could be 'complexified' so much that it needs to run through the ECU. (and still the ECU doesn't tell you that the lights aren't working !)

 

therock88 - How long do you think it would take to reset ? I put everything back to normal (with traditional bulbs), waited ~10mins and there was no change. I was thinking of disconecting the battery to force a 'reset' ?

 

10 Minutes with the lights on should have been sufficient. Assume you had the car on and lights on for that time? Maybe drove around? It could be something more serious I suppose, but I do know there have been a number of cases where it just took time. I suppose you could try to disconnect battery. Before that I would drive around with the lights on for up to a day ten check.


If you want to ensure everything is cleared in all modules...Disconnect both + and - cables and clamp them together for 5 minutes or so...This will discharge any memory stored in any of the components.


Hope that works. If not, sounds like you may have a more serious issue since you were not getting power to the rear harness at all?


Good luck...report back so I or others can continue to try and offer thoughts (if any additional).


DC

 

Ok.... so I drove around with lights on full for greater than 20mins.... still no light to the license plate. I connected the two battery cables together for much greater than 5 mins, no luck.

At this point I'm completely out of ideas... (but really apprechiate the help).

Is there another physical location where I can check the voltage etc, which could lead to some insight as to what's going on ? Otherwise, I'm thinking it's time to take it in and have it looked at....

Thanks!

 

I wanted to provide the final outcome for the use of others.

After some diagnostics it turns out that a diode blew in the rear lighting control module (under the drivers seat) which prevented power from being sent to the license plate lights.