Trying to understand "Service Soon" and Number 28 Diagnostic code, '85 Seville

[joeinbcs]

Hey there,
My, new to me, '85 Seville has exhibited some behavior that I couldn't find explained exactly in the FSM.
The "Service Soon" light comes on when I first start the car, but dims to the point of barely being able to see it shortly after.
A check of the diagnostic codes revealed that Code 28 displays, which indicates an issue with " Hard DFI Code 28, Open Fourth Gear Circuit".  The car seems to operate as designed and shift into all gears when appropriate.
There's a fairly complicated diagnostic procedure to determine if the problem is related a faulty ECM, or a faulty Transmission switch, or just an intermittent gremlin.
Just wondering if anyone has experienced this issue with their car?
Thanks, Joe