If the alt is putting out 13.8 volts and the battery is only at 74%, I think I would put a battery charger on the battery for a couple hours and retest it. At 13.8 volts, the battery should be very near 100%. I'm assuming the battery terminals are both clean?

These computer controlled motors don't do well if the battery is not up to par.

If after a recharge, and the battery tests good, if you still get the low voltage code for the TPS, you might want to check the connections for a tight and corrosion free fit. And clean out the carbon out of the TPS passages real good. Gene