I recently bought a 2005 X type 3 litre and I have noticed that about 20 minutes after driving the car with the touch screen on it starts to darken until it is barely visible, making it virtually unusable. The screen brightening and contrast functions have no effect. Does anybody have any ideas what the problem could be, I'm hoping it may just be a matter of changing the bulbs, here's hoping.

One possibility is that the ambient light sensor is breaking down. To test if it is working, cover it and see what changes of screen brightness occur.

 

If you can't replicate the problem like that then the problem almost certainly lies with the screen.

 

Another way might be to disconnect the ALS entirely but I'm not sure whether this would darken or brighten the display Graham.