My story about dash board warning light:
I bought my car 4 months ago with the Exterior light failure on, I thought no big problem but spent almost 4 months looking for the cause of the warning. All exterior lights were OK, no corrosion at sockets or so. I gave up then and accepted as a minor nuissance.

Then after a signal light was defective, the Check Engine light went off when headlight was turned on. After fixing the light CEL went away but Exterior light failure waring was still there. I posted my question here and as David correctly pointed out, the bulbs for Exterior light failure and CEL were switched. That means I have been driving with CEL on the whole time without knowing.

It turned out to be the Coolant temperature sensor was defective. I fixed the problem including switching back the warning bulbs and have the CEL no more but now I am not sure whether the seller had intentionally switched the bulbs to cover the CEL warning.

My experience for the next purchase is Be suspecting of any warning light on the dash board.

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1996 E320 since 1/16/08, 171K miles as of Feb 2011

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1989 300CE - R.I.P. Dec 29 2007
Other MBs (sold): 1992 300E-24 - 1979 350SLC - 1984 230E - 1990 300CE