Had the same problem a few years ago. See my fix here - http://www.peachparts.com/shopforum/showthread.php?t=86110&highlight=economy+light

Some say you only need to recharge the system and maybe add some dye to find the leak. MB says you always must replace the sensor and reprogram the unit as indicated in my thread. A common leak area is the short one foot hose from the receiver/drier to the compressor. Look for oily residue at the R/D end.

BTW, I never had to add any R134 to mine, but I'm now noticing that the driver's side is a little warmer than the passenger side and there might be a little leak at the hose I indicated above. I suspect I'm now due for a recharge.

Len