The rear air cylinders on the e350 are collapsed, and the dash warning is on = "Stop. Car too low" When the car is unlocked, you can hear the compressor going, but nothing happens, the rear is still slammed. Common problem? What is the most likely part that needs replacing?

So the car is a E350 Touring?
The standard rear only air springs are a somewhat common failure.

NOT hard to replace, if you wait...the pump will be Shot as well! $$$

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but they would both go at the same time?