Use cruise RPM, commanded lockup data, and ratio behavior to separate no-lockup converter-control faults from top-gear issues and true internal transmission slip.
If highway RPM stays a few hundred revs too high, the transmission feels like it never fully settles, or fuel economy drops mainly on steady cruise, treat that as a lockup-routing clue. The first question is not whether the converter is dead. It is whether the TCM is commanding lockup, whether the clutch applies, and whether the transmission is even reaching the gear where lockup should happen.
P0741, P0740, P0743, and P2761 all keep the converter-control branch near the front because they accuse the lockup command path more directly than a generic ratio code does. When commanded lockup never arrives, actual converter slip stays high at cruise, and the highway RPM complaint appears without a dramatic flare on gear changes, think TCC control, case-connector trouble, internal harness issues, or a valve-body solenoid path before condemning the whole unit.
A vehicle can have high highway RPM because it never reaches overdrive, not because the converter clutch failed. That is why commanded gear, actual gear ratio, input speed, and output speed matter. If the transmission never gets into the expected top gear, ratio and shift-control faults deserve equal attention. If the gear is correct but converter slip never drops, the lockup branch is stronger.
Real internal slip usually gives a broader story than no lockup alone. The driver may report flare during shifts, delayed engagement, burnt fluid smell, or ratio codes that return even after electrical checks pass. In that case the converter may still be involved, but the problem is no longer a clean lockup-only branch. The data should show believable commands with an outcome the hardware cannot achieve.
Poor highway fuel economy is one of the simplest real-world clues for missing lockup because the engine keeps turning faster than it should on a steady road load. That is different from poor city MPG caused by warm-up faults, rich running, or misfire. If the complaint is specifically highway mileage dropping together with elevated cruise RPM, transmission lockup belongs much higher on the list.
First, road-test with scan data and capture commanded gear, commanded TCC state, actual converter slip, engine RPM, input speed, and output speed. Second, inspect fluid condition plus case-connector and internal-harness evidence because fluid intrusion can fake expensive failures. Third, prove the TCC circuit and solenoid path on electrical codes like P2761 or P0743. Fourth, if commands are credible but slip stays high, widen the diagnosis toward valve-body wear, converter clutch failure, or internal hydraulic leak with confidence.