After contributing to this website for a year or so, it seems to me that many problems with cars today (not just Jaguars) involve microprocessor control of various functions. I speak here as one with around 50 years experience in the electronics business, but only as a heavy user of cars and as a DIY-er when it comes to fixing them.
A big change in the electronics business came about with the digitization of many procedures in the professional sphere, e.g. in the 1970s most recording studios went to 14bit digital recording for making master tapes. As prices fell, the domestic sphere became involved.
Another change was the microprocessor control of machinery, e.g. now your humble washing machine has microprocessor control. This has resulted in ordinary cars being continually controlled as to various functions, even autoboxes being controlled by a "learning" program which adapts to the driver's habits. This is not particular to Jags. Common cars have this feature too.
We now have "software" as an important operating feature in the cars we drive. This results in rather strange behavior sometimes, also found in other microprocessor controlled devices. We are getting this 'strange behavior' in this very website, e.g. the appearance of faults like "restricted performance" when no discernible cause can be found. Also the infamous "lurch" which has spawned its own website even.
The approach to curing these software/microprocessor errors has to change from the old mechanistic thinking that was used to fix cars that had no such control. It's just not real-time anymore. Instead, more empirical methods need to be used (suck it and see if you like). Very often, there seems to be no logic as to what is causing the problem. So the good old system of "logical fault-finding techniques" that I was taught at technical college no longer applies in the microprocessor world.
So what is the upshot of this?
1) Always go for the simple/easy/cheap things first.
2) Don't jump to any conclusions about what's the real problem. Keep your mind open to any possibilities, no matter how illogical it seems.
3) A disproportionately high number of what look like microprocessor problems aren't micro. problems at all. There is a problem with it's power supply is all. In cars, this is simply the battery and or its wiring.
4) In the micro./digital world, there is great sensitivity to "glitches", i.e. electromagnetic power 'spikes' both along wires and through the air. These are often caused by sparking connexions or by actual sparks as in spark plugs.
5) Manufacturers make mistakes in their micro. equipment, hardware and software, which don't show up until months/years later.
6) Bear in mind that errorcodes read out from your micros. by a scanner may have absolutely NOTHING to do with the problem you are experiencing. The ercos. can be stimulated by glitches when there is no such fault. Equally, the erco. could be right! You just don't have enough info. to know for sure.
So if you are getting baffled with a strange, illogical problems with your car which defies all attempts to cure it, you just might have some idea why now.
Leedsman.

 

Leedsman, that is some good advice. I also find that if you can associate different aspects of the car with a problem, it can help you narrow down where the problem most likely is. Silly things like having the radio go out and then finding out that your power windows don't work either. Most people would think they are 2 separate issues when it reality it is simply the cars way of telling you there is a problem with the GEM module. So, being able to understand what the car is saying through what it does can tell you tons of information. hence why there are times where I will ask people about different aspects of the car that don't seem to be related to the initial problem.