Hi, bought a 1999 XJ8 3.2 about 3 months ago. Very good condition inside and out. Have had a couple of restricted performance episodes, which have been cured by cleaning contacts under the bonnet. Car was running fine yesterday, this morning I went out and unlocked the car with the key, held over to wind down all the windows, (it gets hot here). About 2 hours later, tried to start the car, nothing. Key will turn to the start position, but no dashboard lights at all. Nothing electrical would work; lights, horn, boot lock, radio. There was a small light on the front of the gearshift surround which was flashing,(I had never noticed it before). Got into boot with the key, voltmeter read 12.5v on battery.
Did some research on forums, could only come up with high power protection module, but that was showing voltage on either side of fuses. Tried to start car , success.
Apparently the red flashing light near the gearshift indicates that the security system is still armed; so to cure this, would need to arm and disarm the car. Yes, I have read the security section of the owner's manual twice; no, I don't claim to understand it.
thanks
Pete

 

?? It starts but everything ELSE is dead?

 

I have had a similiar experience with everything going dead. I sat in the car thinking about the problem for a couple of minutes, tried again and it all worked fine. I heard somewhere, if you turn the key to fast it can avoid detecting one of the key positions and invalidate the security by shutting off the car.
So now I turn the key slower when starting and stopping the motor and the problem has never reoccurred.
Worth a try.