Lots of things can cause a shift fault to show up on your engine, but without the actual fault(not what’s displayed on a gauge) it’s tough to say. Does the fault go off just at key on, engine running not shifting, or engine running while shifting? The DTS system can be sensitive to voltage so it would be worth making sure all your battery connections are clean and tight as well as the power and grounds on the engine.
Honestly I would find a dealer with the actual Mercury diagnostic computer so you can view fault history. The fault will be there. The electronics for DTS can be very hard to diagnose without the fault, and the computer hooked up to watch what the actuator and control are doing.