The EC button is EConomy mode. I have an intermittent problem.

When I press the EC button sometimes the red light comes on bright and
the aircon is shut down and when I press it again the light goes off and
the aircon comes back on. This is as it should be.

Sometimes, however, the light only comes on dim and the aircon shuts
down but I cannot get it to come back on by pressing the button again.
It seems to be stuck in some intermediate state. Leaving the car for a
day or so seems to restore it to a working state.

Obviously the compressor must be working since the aircon gushes cold
air when everything is working but there must be a valve that sticks or
a contact that is corroded. What are the other components to the aircon
system on a year 2000 E320 cdi?