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F21 is hot ONLY when cranking. It would be extremely long odds that the starter relay is causing the intermittent short A wiring harness is a far more likely problem. The starter motor itself cannot cause this problem.
Have you had any exhaust or transmission work done? The harness for this circuit is easy to damage when doing this type of work. Otherwise, in might be damaged due to chafing or rubbing.
It is necessary to identify your vehicle including year, model, engine, and transmission since there are considerable variations between the different versions. Also, identify if any aftermarket alarms or security systems are installed.