1998 XJR

I have a Jaguar XJR with about 71000 miles on it. Last week my amber "check engine" light came on with no clear problem. I called my mechanic (and father-in-law), and of course he said to bring it down and he'd read the code. I couldn't get down there for a few days. The light went off the next morning. A day or so later it began to run hot, and to make a long story short my water pump died. I got right off the highway and had it towed. He replaced the water pump and all was well. Until the amber "check engine" light came back on, then for a split second all the warning lights came on, then went off and everything was fine. I brought it in, had the code read and it was a P1638.
I looked it up and all it says is "CAN link ECM/instm curcuit network." My question, what does this mean, what do I do, and what will it cost?