EC light stays on. Please help
 

I have S420 99 and the ec light on the AC panel stays on and doesn't go away. Please advise what to do and what can be the problem.
Thank you

Of course when the EC lamp is on there isn't any A/C to help wit window defrosting. This can happen due to several problems, including low or no pressure in the system. Also there is an overheating switch in the drier.
Reading the computer codes will be the best way to find out.

The codes can be had from the pushbutton controller on most models. I wrote an article describing the technique on a 210 chassis car E320: http://www.continentalimports.com/ser_ic40142.html

They should be similar to yours. If you read the article and want to try I will list the method here for the S420 just in case it is different from the article.

Turn ignition on, Set left display "Hi" set right display "Lo", within 20 secs press the "Rest" button and the "EC" button simultaneously for greater than 5 secs. The LED in the "Recirc" button flashes and the display window reads "Off". Press the right "Auto" for greater than one second. If there are not DTCs (diagnostic trouble codes): "E" is displayed in the left side of the display window and "FF" is displayed in the right side.

If DTCs are stored in memory : Ebl is displayed in the left side and the DTC (e.g. 416) is displayed in the right side.

By pressing "Auto" the next code will appear. Do this till all codes are read.