This is what i found in my pdf diagnostic info for that code.
System Voltage – P1793, P1798
A permanent supply is used to maintain a battery backed 'memory'. Should this supply be cut, due to battery disconnection perhaps, the 'adaptive shift' values will be lost. This will result in a small reduction in shift quality for a period until the adaptions are 're-learned'.
The TCM monitors the supply voltage and will log a Diagnostic Trouble Code (DTC) if the voltage is not between 7 and 16V for more than 300 ms whilst the engine speed is greater than 1600 rpm. This fault is unlikely to be caused by a failure within the TCM and as such the vehicle harness and fuses etc. should be checked before the TCM is condemned.
I dont know what year your car is, but that applys to "ZF 5HP24" XK8 4.0 L N/A 1997 to 2002 model year
good luck. keep us posted on your repair.
My baby... God i love that sound...
PLEASE NOTE: This exhaust was NOT made by Nameless Performance. Only the High Flow cats were made by Nameless Performance. My exhaust is vary aggressive. I like it that way. That does not necessarily dictate what the cats it will sound like on your car.