2004 XJ8 4.2L, 104,000 miles.
I recently got the dreaded "Air suspension fault", followed a few days later by "VEHICLE TOO LOW", with the rear right corner of the car now half an inch lower than the other corners.
The distance from the ground to the bottom of the wheel-well (not counting that flange that runs around the underside) is 5.75" for most corners, and 5.25" for the right rear.
When I start the engine, I hear what I think is the compressor running, but the suspension lifts only a tiny bit, like 1/4".
It's still driveable and still gets the same gas mileage, but it is a bit lower than usual.

I found no error codes with my ordinary $15 ODB2 scanner, just P1111 (scan complete).
I bought a Foxwell NT510 code scanner, and it found 60 non-OBD2 codes.
(The NT510 identified my car by VIN as being of the year "2002.5". Does that mean it was manufactured in May of 2002, or is the NT510 just wrong?)

What should I do???

A lot of them say "short to ground". Possibly there's a single short somewhere causing many problems.
Also, there's "Generator charge system failure/generator 'load' feedback circuit failure" under ECM, and later several "battery low" error codes.

I've put them into csv format so that the text file can be loaded in Excell, OpenOffice sheets, or Google sheets.
I loaded them into a Google sheet here, which you can read if you have a google account, and you can edit the columns to the right of my comments to leave your own comments.

Lines beginning with a 0 give not error codes, but a module name, indicate the start of a new module.
Lines beginning with a 1 give an error code found by the module named by the most-recent line beginning with 0.
A couple of codes are duplicates, listed under 2 different modules.
After the leading 0 or 1 is a comma and another number, where, for lines starting with 1,

• 0 = I don't care about this issue
• 1 = I wish I knew what this code meant
• 2 = This could be important
• 3 = This is definitely important

That's so you can sort A-Z on LINE# to see them in order, or Z->A on "0 ignore, 3 critical" to see in decreasing order of hypothesized importance.

0,2,ECM,Engine control module,8
1,1,P0860,Gear shift module communication circuit
1,1,P1260,Security input
1,1,P1582,Flight recorder data is stored
1,3,P1632,Generator charge system failure/generator 'load' feedback circuit failure
1,3,P1672,CAN link; engine control module to air suspension module network malfunction
1,1,B0300,Network management expulsion
1,0,B1316,Boot/trunk latch power close unit,NEVER LIKED THAT FEATURE ANYWAY
1,0,P1111,System check complete
0,1,IP,Instrument pack,15
1,0,B2883,Steering column tilt movement fault--primary axis
1,1,B1602,Passive anti-theft system received invalid format of key from ignition key transponder
1,1,B1202,Fuel sender: circuit open
1,0,B1875,Turn switch left and right switches active,TURN SIGNALS WORK FINE
1,1,C1922,Power steering solenoid actuator circuit - open,POWER STEERING WORKS FINE
1,1,U1900,CAN communications bus fault
1,3,B1318,Battery voltage low,BUT I JUST CHARGED IT COMPLETELY
1,1,U2200,Invalid data for rolling odometer count
1,1,B2139,Data mismatch (received data does not match expected data)
1,0,B2884,Steering column tilt movement fault - secondary axis,HAVEN'T MOVED STEERING COLUMN IN YEARS
1,0,B2881,Steering column reach movement fault - primary axis
1,0,C1986,Variable assist power steering initial speed is above threshold.,DIDN'T MOVE CAR
1,1,U2510,CAN - invalid data for vehicle security.
1,1,B2879,Fuel tank jet pump fault
0,3,ASU,Air suspension module,5
1,1,C1419,Front right damper - circuit open.,DAMPER = SHOCK ABSORBER?
1,1,C1424,Front left damper - circuit open.
1,3,B1671,Battery module voltage out of range.
1,3,C1830,Air suspension compressor relay - circuit failure
1,3,C2302,Levelling plausibility error.
0,2,ATC,Climate control module,3
1,0,B2513,Blower fault,BLOWER IS FINE
1,0,B2514,Blower fault,BLOWER IS FINE
1,1,U2520,Missing message from instrument cluster
0,0,DDM,Driver's door module,3
1,1,U1262,SCP (J150) Communications bus fault
1,3,B1676,Battery voltage out of range.
1,0,B1895,Driver's/passenger's door ajar output - circuit failure.,I DON'T THINK IT WAS OPEN BUT AM NOT SURE
0,2,DSM,Driver's seat module,2
1,0,B1940,Seat memory position out of range.,NEVER USED THAT FEATURE
1,0,B2309,Seat motor out of range.
0,2,GEM,General electronics module.,13
1,1,B2363,Optical sensor system failure,???
1,1,B2496,Anti-theft horn circuit - short to ground
1,3,B1676,Battery voltage out of range.
1,1,B1838,Battery saver power relay - circuit failure.,COULD THIS CAUSE MY BATTERY PROBLEMS?
1,2,B1437,Windshield wiper high/low speed relay coil circuit - short circuit to ground.,REPLACE RELAY, OR FIX SHORT-CIRCUIT FIRST?
1,2,B1433,Windshield wiper brake/run relay circuit - short circuit to ground.,REPLACE RELAY OR LOOK FOR SHORT-CIRCUIT?
1,2,B2414,Headlamp washer relay circuit short circuit ground,WHAT'S A HEADLAMP WASHER?
1,0,B2598,Headlamp relay - circuit failure.
1,2,B1570,Headlamp high beam circuit short to ground.,HIGHBEAMS SEEM FINE
1,2,B2030,Front fog lamp relay - circuit failure.
1,1,B2262,Windshield wiper park to park timeout,???
1,2,B2315,Passenger's mirror horizontal feedback potentiometer circuit short to ground,MY PASSENGER MIRROR GETS OUT-OF-POSITION EVERY TIME I PUT THE CAR IN REVERSE
1,1,B1987,Pedal forward/rearward motor stalled
0,1,REM,Rear electronic control module.,9
1,1,U3FF6,Parking aid - short circuit to ground.
1,0,B2533,Rear right stoplamp - circuit failure,RIGHT REAR BRAKE LIGHT FINE
1,1,U1262,SCP (J1850) Communications bus fault.
1,1,P1235,Fuel pump control out of range.,NO OBVIOUS PROBLEMS WITH FUEL SYSTEM
1,1,B2162,Right side module invalid identification.
1,1,B1221,Left seat heater output - open load.,I DON'T THINK I HAVE SEAT HEATERS
1,1,B1222,Right seat heater output - open load.,I DON'T THINK I HAVE SEAT HEATERS
1,0,B2526,Rear left back-up lamp - circuit short to battery,NO PROBLEM WITH BACK-UP LAMP
1,0,B2577,Left tail lamp circuit failure.,NO PROBLEM WITH LEFT TAIL LAMP
0,2,FETM,Audio unit,1
1,0,U2607,Slave switched into bypass mode.
0,2,PAM,Parking aid control module.,1
1,1,C1700,Rear left corner sensor - circuit failure.,MUST REPLACE LEFT REAR BACKUP SENSOR?
0,2,AMP,Amplifier,2
1,1,B2642,Low power detected.
1,1,U2609,D2B wake-up pulse width out of specification.

 

Never mind. Massive error codes were due to a low battery.
Air suspension problem located: