First, scan for trouble codes, that can indicate the problem or provide some insight, Verify the compressor is running. If so, use the age old troubleshooting method of soap bubbles starting from the compressor. Follow the lines to the air suspension "bags". In some circumstances the troubleshooting won't expose the problem necessitating re-setting the factory levels using a scanner. You can also use the scanner before troubleshooting for leaks.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas