I just got a 2011 XJL supercharged from auction. Listed as "runs and drives" but that was not correct when the car was delivered to my home.
When we tried to start the car, we got a "low battery voltage" error. We changed to battery to a brand new battery.
tried again and this time the ignition will turn on but we get a click noise once and then nothing.
we plugged in the Fixd sensor and got error code U0122.
Looking for the starter relay to check that before we buy a new starter. can anyone help me with this?

I am at a loss on this car already. The dealership told me it's relay#10 under the hood.. but there are only 8 here.

 

Get used to the "low battery - restart car" on the dash if you leave the door open for a couple minutes or anything more than that.
That's just characteristic of the car, particularly 2010 through 2013 (IIRC).
As far as the starter relay, I do recall a few posts asking related questions, so if you go to the "advanced search" tool in the forum & type stater and/or relay in the search (specifically for X351 forum) you should be able to get some helpful narratives & references.
Hiopefully it's something as simple as a relay.

 

The start relay is not a plug in type but mounted on the fuse box circuit board. AFAIK is not accessible/replaceable.