Which of the vertical buttons on the right are you using. The most assured AC comes from the upper button that looks like defrost. Using te center button requires the mode door to go all the way closed before the AC will turn on. In the upper button position the AC is on in any setting.

If that doesn't matter, check to see if the compressor comes on, if not evaluate the low pressure switch.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician