Climate control problem
 

I have a 1999 E320 ,the EC or economy button stays lit all the time no matter which button you push ,except for the defrost which shuts this of .My question is is this an indicator of a specific problem and is there a button sequence that I could initiate a built in self test to identify the problem.

either the refrigerant pressure is low or the temperature sensor got problem.

I lowered the temp to kick in the compressor ,but it did not i guess this is a refrigerent problem,Thanks