Thankfully there is 1 levelling sensor to the rear, its plastic. Worth visually checking that in case it's broken. I don't think there is a way of testing the actual sensor so I reckon if it looks intact, just move onto the block. The fact that the compressor has fired up, it does seem to suggest a possible problem with the block, maybe a stuck valve?
Getting warmer and I think you'll be an air suspension expert very soon