nigelharley,

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.

 

I also had a reading of gearbox fault, I believe it is the transmission, it has to be re-built or replaced. Does anyone know how much that would cost?
My car is sitting at my home and, it will not start anymore, I have enjoyed driving my car for two years and, am having a hard time getting rid of it.
Offered only 800.00 dollars..I was told also it could just be the timing..
I am not sure what to do now. ; ( it is a XJ8 2002...

 

I had my transmission in my '99 rebuilt 3 years ago for around $5,000. It lasted 40 thousand miles and broke. I am going with a replacement from a junk yard for $3,600 installed. Not happy with rebuild!