04-16-2012, 10:27 AM
Date registered: Sep 2006
Vehicle: 2004 CLK 240 Coupe
Location: Norfolk. UK
I had the same experience about 3 years back. On the W209 the cluster message identifies the failed lamp - except that it was working fine.
Some weeks later, one licence plate bulb did fail to operate, but on inspection the bulb was OK and just cleaning the contacts fixed the problem.
So I expect that all your bulbs are OK on the full working voltage, but one of them throws a false fail when tested by the lower diagnostic voltage. Not much you can do, unless you want to clean up the contacts of all the exterior lights...
You could just try cleaning the licence plate bulbs contacts, and that might fix it. That area get more exposure to the weather than the other lamps.