Just a little update, its been about 5-6 weeks since the replacement of my J-gate and all is still good. So needless to say I'm happy

 

Well after about 8 months of everything being good suddenly all is not so good again All the warning lights are back as described in the original post. I've taken the vehicle back to the dealer and they said they had some low voltage faults but could find nothing else wrong. The battery passed load testing (it was replaced last year when this problem started) and the alternator's voltage and amperage is good. Needless to say I'm at a bit of a loss as to what's going on here.

 

Update, problem has been ongoing since last update. I have made one discovery: when the dash panel lights up like a Christmas tree I can kick the under dash panel and all starts working again (but I'm left with a check engine warning). If the problem doesn't return within 2-3 days the check engine warning goes away. Real problem right now is my smog is due and before I can get the monitors to set this whole vicious cycle starts over. I've been under the dash and didn't locate and wires that looked to be damaged. But there was evidence that the wire harness had been played with as the wrap tape was missing in many places and the harness holders broken (or missing). So I taped up the harness and used zip ties to secure the harness. Its been 12 days and all has been OK, plus I finally have code P1111 so I'm clear to get a smog test. Hopefully this will be the last of this problem. Later this year I'm planning on selling the XJ8 and have been considering the newer XJ (X351). Hopefully I won't be jumping from the frying pan into the fire, LOL.

 

Not sure if this is the right place to respond but I would suggest that damp/moisture in the circuit board beneath the J-Gate can cause all these warnings to appear.
Mine is a 2004 XJ6 that's been running fine. I live in Hong Kong and once last summer though and again last night, we had heavy rainfall which I believe came in through the sunroof and leaked down from the front of the headlining by the sun visors. Last summer it pooled in both footwells whereas last night, there were traces of water by sun visor and on the veneer surround the J-Gate.
Car started perfectly but would not move from "P". I turned the engine off but could not withdraw ignition key. Went back a few hours later, car started and actually moved out of "P" but before deciding to drive off, all the warnings appears such as DSC Not Available, Brake Fluid Low, Gearbox Fault etc. I then turned the engine off and could this time remove ignition key.
Fifteen minutes later, went to start car again. This time, all the warnings immediately there, engine overheat light on also but nothing when I turned ignition key. Tried disconnecting battery for a while but that had no effect.

 

Excessive moisture anywhere that is supposed to stay dry.
Can cause all sorts unpleasant electrical issues.
Finding & fixing the cause, is a paramount issue, I M H O

 

Mine stopped doing that after I changed the batteries in the key fob. It was doing the whole bit; cascade of faults,shifter locked in park. I did that simple thing and it quit all of that. At the very least it is a cheap thing to try.