This is what I've read about this problem... found it on a forum couple of years back
'...to remove the "check contol inactive" message you have to go to a BMW dealer or a garage that has the BMW tester ...your OBC has to be reprogrammed , no big deal ... estimated cost 10 to 50 USD, make sure you tell the guys about the OBC message...'
Did you attempt any changes to your main unit? The code cannot be cleared thru the console, your computer. This error code is stored in your cars memory not the OBC's. The dealer will plug in their diagnostics and will reset all the fault codes that appear. This will solve your check control inactive. This is a common problem on many 1992's... I don't know why... ghost in the machine.... but I may add fooling with your car's software may produce the same problem.
hope this helps.