DTC C1804: Height Adjusting Malfunction (Compressor): Description
WARNING: This page is about a different car, the 2013 Nissan Armada. However, it is still accessible from the selected car via links, so may be relevant.
The compressor motor relay is controlled by the suspension control unit. The suspension control unit supplies voltage to the coil side of the relay to activate it. The compressor motor relay, when activated, supplies power to the suspension air compressor.