Valve and compressor was the issue for me. Took about 2 days since the workshop had the parts. $11XX including labour.
I have the W204 FL , made in 2013, and had this issue for damn long. So anyway, went to a renowned car workshop in Sin Ming and they did checks and said no issue. So went to a workshop who specialised in mercs upgrades in Toh Guan, and solved the aircon issue for me.