Most likely the expansion valve in the rear unit. Front unit uses an orifice tube. You can get to the A/C lines for the rear unit by crawling under the vehicle where the unit it (I think it's the left hand side) and feel the lines to see if the suction line is cold. If not, the expansion valve is clogged or failed. Make sure there isn't debris in the system, such as from a failed accumulator.

A less likely failure is the temperature blend door no longer operating and being stuck in position.

Either way, the E-Series dual A/C units really don't cool all that great in hot weather. 60°F at the vents is considered acceptable according to the manufacturer temperature-pressure charts for normal operation.

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Erich Loepke
2010 Ford Focus
Currently Benz-less