E240 AirCon not working properly

Hi,

Each time I start the engine & then start the air con it will either not work properly or not work at all. It been this way about a week now.
If it's not working properly, is where, it will not respond to increasing the A/C to maximum or it will go to max but will after a few minutes adjust down by itself to half the fan speed.

On many occassions, the A/C will not come on at all. Putting on E (economy) also have the same effect.

Can some 1 pls help me to diagnose? Is there a control unit or fuse I need to check or can be easily replaced? Where should I check?

Thanks


RMT6668