BMW Repair: E39 Air Conditioning, uv dye, compressor clutch

Question
The A/C on my 528i suddenly stopped blowing cold while the car is idling, but still blows cold when the RPMs are above idle level, though not as cold as before.  This was a sudden change (not gradual) that was accompanied by the Check Engine light coming on.  I talked to my mechanic on the phone who thinks the two issues are unrelated, but they happened at exactly the same time.  I haven't yet hooked the car up to the computer to get the fault codes.  Any suggestions as to what might be happening?

Thanks

Answer
David, Is your A/C compressor clutch engaging and spinning with the rest of the pulley? Does your outside temp. sensor work correctly on the cluster display. If it is broken an reads -40 then it will disallow compressor operation and cause check engine light. If this is okay then have the fault read out of the DME for the check engine light. Also should recover the refrigerant to see if you have lost any and if the correct level was maintained. Add some refrigerant oil with UV dye in it if the system was found to be low on charge to assist in leak detection.
-Evan