Two questions:
Are there any external switches,sensors, or fuses that would result in "gear box failure"....?

Is the 2000 xk8 a Ford transmission?

 

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Just to get you started in your research, the transmission in the earlier 4.0 XK8s is a 5HP24 by German manufacturer ZF. There are logical diagrams out there that describe the various sensors as well as inputs from other modules. This transmission was used in several cars, so you might have hits for BMWs, etc.

Generally speaking, these transmission have fairly sophisticated diagnostics capabilities and produce codes that help with the specifics. If you are experiencing problems, these codes are a good place to start. Several places offer to read codes for free.

Best of luck, keep us posted.

 

Welcome to the forum. One external source that could cause your problem is a low or weak battery. If it's fully charged it should read at least 12.5V after sitting overnight. If in doubt have your local auto supply do a load test.

They will read codes, but many transmission codes are Jaguar specific and you may need to visit a foreign car specialist or possibly transmission shop.

The dealers can of course, read the codes, but they have a tendency to want to replace everything to see if the problem goes away, and you're stuck with a bill higher than the value of the car.