Drivetrain Malfunction

Most of the time I drive within or close to electric range. Every now and then do a longer trip….last week drove 200mile R/T from Brooksville to Sanford to pick up an '04 Jag Vanden Plas….or like tomorrow I'm driving to St Pete….when the battery gets to less than 75% I turn on the range extender. As you know driving 75mph at times requires both the REX and the battery. Every time I do this towards the end of the trip I get a warning "Drivetrain Malfunction" get home, after a day it goes away and all is normal.
any ideas? even after the warning comes on the car drives normally, but still stressful!
Dan

It happened to my 2014 i3 twice before it was out of warranty and the dealer never and record of it because it was gone by the time I got it to the dealer. Then when ti was 9 months out of warranty it came on again and stayed on, i took it to the dealer and after having the car a day looking for the problem they called me and said it was a heat exchanger of some sort and the cost was estimated at $2800 to replace it. I only had 9650 miles on the car and I complained about the failure and the dealer got BMW to pay for it.