I do not even have a ZONE button on my climate controls... @Rickman30 are you talking about your 2014?[/QUOTE]

Yes, my 2014 E350 calls the dual zone climate control button that syncs or un-syncs the desired temp control "ZONE". On the W213 I believe it is the "sync" button.

 

Yes, my 2014 E350 calls the dual zone climate control button that syncs or un-syncs the desired temp control "ZONE". On the W213 I believe it is the "sync" button.[/QUOTE]

Gotcha, yes the SYNC button is what my car has. I leave my car in AUTO and leave SYNC on almost 100% of the time unless I have a passenger, as most people don't like the 65-degree temperature I keep my side at.

 

Alrighty y'all, the final update is here. I was able to evacuate the A/C system and replace the refrigerant pressure sensor with one I bought on RockAuto. While I was in there, I cleared a sizable amount of leaves and other dirt from the space between the A/C condenser and the radiator. I then recharged the system and it works perfectly. I cleared the refrigerant pressure code and the low refrigerant code that appeared when I started the engine after evacuating the system. For some reason, this also stopped the engine cooling fan from running at full blast, perhaps because of the cleaner radiator but also likely due to something changing after the A/C started working again.

While filing my paperwork from the most recent visit to the Mercedes visit, I noticed that a previous service visit had also turned up the same thermostat code, but that paperwork long predated the engine fan complaint. Since the fan has stopped screaming and the car exhibits normal behavior, I am thinking I will leave the thermostat alone. The dealership did not recommend that I repair that issue when that code was present before, and now that I have resolved the principal complaint, I'm likely not going to spend the money to fix it since the car is asymptomatic.

Thanks to everyone who helped here - your advice was valuable and made repairing this issue doable at home with some rented tools and a little patience.