It's low on freon, assuming this is the 99E320. Mine does this every spring. Very, very small leak, somewhere. Just enough to require a small charge and she's good to go for the summer. But the symptom is cool on one side, warm on the other.

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DS
2010 CL550 - Heaven help me but it's beautiful
87 300D a labor of love
11 GLK 350 So far, so good
08 E350 4matic, Love it.
99 E320 too rusted, sold
87 260E Donated to Newgate School
www.Newgateschool.org - check it out.
12 Ford Escape, sold, forgotten
87 300D, sold, what a mistake
06 Passat 2.0T, PITA, sold

Las Vegas NV