2012 xj x351 rear sits on tires. Really don't know where to start when it comes to troubleshooting this issue.. I tried driving for a while hoping air compressor would pump up the rear. But it doesn't even try to kick on. Any insight would be greatly appreciated thanks guys

 

Will need to use SDD to find the problem. It's rare to have air suspension problems in this model of XJ to so hope it's something minor.
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Have you got any fault display ? The system works via an Air Suspension Module (ASM), that receives height signals from height detectors for each wheel, and supplies or exhausts air in the air springs until the height signal matches the required height value. An air compressor keeps the reservoir topped up. If there is a fault display, then as clubairth1 says, you'll have to read the fault codes to find out what is going wrong.
The X351 cars only have air springs on the rear wheels, the fronts are steel coils, so less to go wrong. My two previous model XJ6s were air springs on all wheels !