Every now and then when I get back to my car (2000 XJ8 4.0) one of the following situations is true:

1) The car will unlock with the key fob, but putting the key into the ignition does not trigger the seats to move into position. Turning the key produces no dashboard lights, no system test, nothing. Turning all the way to ignition also does nothing, just dead. Waiting a while (20-30 minutes usually) and trying again the car starts normally. The radio reads "enter code".

2) The car unlocks and starts normally, but the radio reads "enter code", suggesting an instance of 1) which has reset itself.

It's as if some sort of protection circuit is kicking in to protect the battery by disconnecting everything, and after a period this protection mode switches off allowing normal operating. Any ideas? I hooked up and on-board diagnostic reader and it came back with P0327 - knock sensor 1 circuit low, which doesn't sound very likely as a culprit.

David

 

Try checking all the main runs of power cables and grounds. In particular, the joint at the false bulkhead on the right hand side at the rear of the engine compartment. It is a through stud with power cables on both sides. There are also multiple joints in the path before and after that joint that need checking.