My 1999 xk8 developed strange problem. After i turn the ignition key on (position II), check system goes on for 10 seconds and than all indicator lights go off. I stll can start and rin the car, but I cannot plug in OBDII scanner, because those lights go off and scanner posts "communication failure" and asks to turn the ignition on. I diconnected the battery for 30 minutes but it did not help. Any idea?

 

Try looking for a bad earth. I would personally start in the boot/trunk. See if there is any corrosion or bad connections. Even try 'wiggling' the thick wires to see if that makes a difference.

I would bet money on it being something simple.