No, the plate is not stuck. It separates completely but is held by the springed assembly.

I was certain that my problem was the duovalve (driver side sticking open). Now that I've cleaned everything, I assume it works fine....but have no way of confirming.

One thing you can try: when the system is running and blowing hot when it should be blowing cold, hit the duovalve repeatedly with a mallet. Oftentimes that will dislodged whatever is sticking and you'll have cold air again.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 160k miles
06 E320 CDI "Rutherford", Black on Tan, 180k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 79k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife