Rear air shocks stuck in the up position

Could be the rear height sensor.

Would need to know what DTC'S - diagnostic trouble codes are stored in the module for the autoride suspension . Hooking up a professional type scan tool ,looking at input data to the module etc... Looking at a wiring diagram an testing electrical circuit's using a DMM - digital multi-meter . Reading how the system works ,what all involved in making the system work . This is how a professional Tech would go about diagnosing this .

The automatic level control (ALC) system maintains a desired rear suspension position under all types of towing, hauling and loading conditions. The following components are involved in the operation of the ALC system:
• Air line tubing--Pressurized air from the ALC compressor is pumped to each of the rear shocks via air line tubing.
• Air drier--Pressurized air from the compressor is run through a drier containing a moisture absorbing chemical preventing water accumulation in the rear shocks.
• Compressor--Supplies pressurized air to the rear shocks
• Compressor motor relay--The relay supplies battery positive voltage to the ALC compressor motor.
• Pressure sensor--The electronic suspension control module (ESCM) module provides a 5-volt reference and low reference to the ALC pressure sensor. The module receives the signal voltage that is relative to the air pressure applied to the rear shocks.
• ESCM--Controls the ALC system and electronic suspension control (ESC) system and detects failures in both. The module monitors inputs from the position sensors, and pressure sensor to determine when to raise and lower the vehicle to trim height as the vehicle is loaded or unloaded. The module limits pump activation to 255 seconds to prevent thermal damage.
• Exhaust valve--An electric solenoid activated by the ESCM to vent air pressure from the rear shocks.
• Left and right rear suspension position sensors--The module provides a 5-volt reference and low reference to all 4 of the body-to-wheel suspension position sensors. The sensors send the ESCM a signal voltage that is relative to the rear suspension ride height
• Left and right rear shock absorbers with internal air chambers and dampers--Dampens the rear suspension and maintains trim height depending on the air pressure applied.

The problem is in the valves that control the height. There will be a small leak in the unit that is causing the fault. Have them /it checked and replace faulty unit

This happens sometimes with dead batteries taht have more than one dead cell in them. Disconnect the Positive battery cable and connect the jumper cables to that directly and the vehicle should start provided the vehicle providing the jump has a battery with enough amperage to turn your motor over. If there is a leak in that front adjustable air shock this will happen again because that compressor will continue to run trying to get the proper pressure on that shock.

check for a leveler adjuster could be stuck or if you are sure that you have front air shocks the line could be leaking hoses cracked or broke or the shock itself could be damaged to check further to find the problem

Shock Absorber -The rear already has air shocks on the 1999 Cadillac Deville: REMOVAL & INSTALLATION Rear

1. Before servicing the vehicle, refer to the precautions in the beginning of this section.
2. Remove or disconnect the following:
   • Rear wheel Fig. 1: Exploded view of the rear shock absorber mounting
     
     
   • Negative battery cable
   • Shock absorber electrical connector from the rear suspension support, If equipped
3. Support the lower control arm with a jack to relieve the tension on the shock absorber.
4. Remove or disconnect the following:
   • Lower shock absorber mounting bolt and nut
   • Upper mounting nut, retainer, and insulator
   • Shock absorber compress by hand and remove through the upper control arm
   To install:
   
5. Position the top of the shock absorber with the insulator attached into the suspension support.
6. Install or connect the following:
   • Upper shock insulator, retainer and nut. Torque the nut to 55 ft. lbs. (74 Nm).
   • Shock absorber lower mounting nut/bolt. Torque the nut to 75 ft. lbs. (102 Nm).
   • Shock absorber electrical connector to the rear suspension support, if equipped
   • Rear wheel
   • Negative battery cable

Check the level switch at the back of the car - this turns the compressor on and off as the ride height changes. It may have come loose, is stuck or faulty.

no,just make sure the little rubber o ring is still on the plastic line before you install on the new shocks,sometimes it gets stuck in the old shock,hope this helps

you might want to check your air control box under car near rear shocks,this starts the air compressor to lift or lower the air shocks

On this model vehicle the rear shockers should have air bags to keep car at normal ride height, when load changes.
The air bag can sometimes become caught between inner and outer tubes of shocker when car is raised and lowered quickly.
You will need to jack car up from tow hitch or frame to lower rear axle, this will unfold air bag, slowly lower car, with ignition on, this will make sure air compressor is filling air bags as they take the load.
Hope this is helpful, MMP