Year 2000 xj8 auto. Not changing gear. Message says gearbox fault and high temperature

 

The first thing that I would do is check the fluid level of your transmission.
It is a PITA but this is the first step to a diagnosis.
Has it been changed recently?
Cheers
RAM

 

Is the gearbox temp ACTUALLY HIGH???

If not then the thermistor integral with the internal harness is faulty.
You will need to replace the gearbox internal harness as the temp sensor is Not Sold/Serviced Separately. (NSS)

bob