I would distrust any warning light on the dashboard.

In my case the warning lights for Check Engine and Exterior Light Failure were switched behind the dashboard (instruments cluster). That means I took the CEL for bulb failure warning, which I thought would be a small issue. After the purchase I tried two months testing around all exterior lights without getting the warning light fixed (because it was actually the CEL) until I got a real signal light failure then the CEL on the dashboard (actually not the CEL) was on.

Now having both CEL and Exterior light warning on I changed the signal bulb and the CEL is fixed. Only then I got supsecting and opened the dashboard and found out that the 2 small bulbs were switched from their locations.

That means I have been driving around with CEL for 2 months. I fixed the problem by changing a temp sensor.

I read about CEL being disabled to cover the problem too.

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

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