2004 XJ8. My car never has any problems. I'm driving along, suddenly every one of the Faults and warnings appear on my dash. I can keep driving okay. Temp says HOT so I pull over. I stop and turn off the car. It won't start up. No dead battery or alternator problems. I did notice my steering got slightly firmer, but no prob really. A/C and windows and radio work. Just won't turn over, and all FAULT warning keep appearing in order. Checked most all the fuses, none damaged. What happened? Can I get this car going by removing the Codes sent wrongly? Also, no air pressure in suspension. I want to get some info before I tow it to the shop. What could cause an instant error on all warning systems when the car was driving fine?

Seems like a low voltage.
Sometimes alternator failure remains unnoticed, since the alt. light won't come on.
Check batt. voltage, if it IS low - replace or charge a battery and drive it home. Check alternator and wiring.

Check earths/grounds too.

Ditto above comments, usually a battery issue or earthing problem, my guess would be check the battery and replace it, have had it happen to mine and the battery change sorted it, best of luck

How long has the battery been in the car? Check the battery voltage, and then have someone monitor the battery voltage while you're attempting to start the car. The voltage shouldn't dip much below 12VDC at that point. If it does, that would probably indicate a weak battery. If OK, then clean the battery posts and clamps. Leave the clamps disconnected for a good 15-20 minutes, and then refasten the clamps. Make certain the clamps are securely fastened to the battery posts. If the problem persists, disconnect and clean all of the earth grounds. Afterwards, secure all of the earth grounds back in place.