2003 Chevy Monte Carlo torque converter clutch circuit stay

Probably it's "open" because he disconnected the plug on the transmission to keep the converter from locking up.

The solenoid goes bad and once it's engaged (somewhere above 45mph) it doesn't want to disengage.
So you come up to a stop sign and the car stalls out. It would be like pulling up to a stop with a manual transmission and not stepping on the clutch pedal.

A lot of people choose not to repair it, as the cost of parts and time involved. It takes a long time to recoop the cost in the increased fuel mileage you get by the 200 300 rpm drop you get at highway speeds when it's working correctly.