Follow-up: Problem solved. My local repair shop replaced both rear air bags (one blown and the other was old w/250K on it). This was caused because the rear linkage popped off, setting a too-low condition; causing it to raise the rear until one air bag exploded and a fault code was set. The local repair shop also said they replaced the rear sensor. After two weeks (seriously!) of not being able to figure out why the rear would not go up, I took the car back from them. I disconnected the rear linkage, started the car, which sets "Ride Height" opposed to "Knee Height", and manually, slowly, raised the rear sensor linkage and then slowly lowered the linkage. The system worked opposite to what was supposed to happen. (When the car raises, the linkage extends higher between the lower A-arm and the chassis. When the car lowers, it collapses, so the sensor arm goes up. Manually operating it in the opposite direction worked.) Repair shop installed the *wrong sensor* in the rear. As soon as I installed to correct sensor, it all worked. For the rear height sensor there is a sensor with a black steel bracket and new linkage available on amazon for $45. Works perfectly.
Additional stuff I learned:
#1: After replacing the two airbags, shop said they were troubleshooting for blown fuses, bad compressor, etc. Claimed they could not get the compressor to come on at all. I told them they either had to clear the existing stored fault codes (since the compressor times out when the original bag failure occurred, or they had to pull the battery cable). They pulled the batt cable. Lesson: Don't expect the system to work after a repair w/o clearing codes.
#2: Troubleshooting a bad airbag. If you manually raising an airbag by moving the rear linkage, you're proving out a bunch of things: Ensuring that the compressor works because it will come one and both rear air solenoids since they will open and close per the linkage movement. If you disconnect the rear solenoid cables after manually moving the linkage, the rears will stay inflated so you can make sure there are no leaks in the bags overnight. The solenoids isolate the air in each bag from the distributed air line.
#3: How it works: I wondered how each bag was actually deflated. There is only one air dump valve and it's located at the compressor, so to lower any one or more airbag, the system must command one or more solenoids to open AND open the air dump valve to vent the air lines.
#4: Repair shop had a Snap-on code reader with NO ability to read, or reset the suspension system code, let alone read, test or set ride height functions (actuate solenoids, read sensor, turn on/off compressor). My Autel MX808 did the job. Best $500 I've spend for our cars!
