lost overdrive on a 4L80E trans code says low voltage to torque converter what do you replace?

Answer 1

You don't necessarily replace anything.
Certainly not without doing some diagnostics.
That is the point of trouble codes; they don't identify a failed component, they indicate an circuit or system that falls outside normal operating parameters.
Low voltage could be either a short or broken wire. Perhaps a blown fuse. Perhaps the TCC solenoid is shorted inside.
There are too many variables and unknowns to just "shoot from the hip".
Locate a QUALITY repair or SHOP manual (library?) and follow the trouble tree for diagnostics. Otherwise, take it to the dealer or a reputable transmission shop for diagnosis, estimate and repair.