My XJR gave me Restricted Performance code and when I went to try and read the system with an OBD2 and it said, "Can not connect to ECU". My engine also was running some what rough and was wondering how to go about diagnosing the issue. Also, was wondering if this could be just a bad battery issue the battery I have is 6 years old and has been recharged a couple times, and my dash and lights are dimming in and out. Only reason I don't want to swap batteries is because I'm curious if there is a bigger issue, and don't want to delete the fault codes.

 

Every cheap code readers does not understand x308. Try another one. Battery Voltage may go haywire. Measure voltages at the battery.

 

The same OB2 worked before it just stopped working as of this issue comming up.

 

That battery sounds suspect and these cars don't do well with low voltage. I would check to make sure you're getting the proper voltage throughout. I've attached an electrical trouble shooting guide; skip the hard reset part (the pages aren't numbered) and go to the one with "First, a Little Electrical Theory." Then do all of the voltage checks.

 

6 years on a battery, that's about the life of them, I would certainly consider fitting a new one. Anything under 12.5V and you'll get ghosts in the machine.