2000 Jaguar XJ8 Failed to start - no codes

Since posting my question, I have taken the car to a mechanic. He diagnosed the crank position sensor as being out of spec - the signal from the sensor was "erratic." He said this device tells the computer that the car is cranking and commands the computer to turn on the fuel to the injectors. I hope this is the real cause and that the problem is now fixed, but only time will tell.

Hi Dave, turns out it wasn't the cause of the fault. Came to start the car yesterday evening, but it only cranked over at the first attempt without firing. A message came up on the mileage screen, "Engine Failsafe Mode". Tried it again and she started. Today, I took the car back to the mechanic and this time he got a code, P1111. He told me this had to do with a faulty idle control valve. Funny thing is my daughter had the same fault on her Lexus RX a few months ago, and after some denials that nothing could be found, her Lexus dealer eventually advised of the same diagnosis. Changing the component on her vehicle cured the problem. My mechanic called someone - a dealership, I believe - and read the part number given by the diagnostic program. The person on the other end of the phone told him the part was discontinued. That sounded weird to me, since the car is only 10 years old. The mechanic then suggested trying to find it on the Internet, or going to a junkyard! At that point, to use a time honoured phrase, I made an excuse and left. Then I called the local authorized Jaguar repair shop. They said they had the part in stock and could fix it, but only after running their own diagnostic. So I left it with them this afternoon. It will cost more than the other guy was going to charge, but in this case it will be well worth it. Will report back on future developments.