What lockup does and why it fails
The torque converter clutch mechanically couples the engine to the transmission input above a threshold speed to eliminate slip and improve fuel economy. Failure modes include fluid contamination that glazes the friction material, a stuck or leaking TCC solenoid, and TCM software that commands apply too aggressively. Each has a different diagnostic signature and only one requires a rebuild.
The scan tool test
Watch TCC slip RPM (commanded input speed minus actual input speed) at steady 45 mph cruise. Zero to 20 RPM is healthy. 20 to 60 RPM is early friction wear or a fluid issue. Over 60 RPM with a shudder is mechanical failure. Combine with fluid inspection: burnt fluid with metal glitter means the converter is done.
When a fluid service saves the job
Early-stage shudder on a transmission that has never been serviced often responds to a quality fluid exchange with the correct OEM ATF specification. Do not use a universal ATF on a modern transmission — the friction modifier package is engineered for the specific clutch material. A $300 fluid service that saves a $4,000 rebuild is a fair trade for both parties.