The primary design function of the OE end plug in listed Toyota/Lexus transmissions is to prevent critical pressure loss by capping the bore. OE end plugs rely entirely on small clearances between the end plug and bore to confine hydraulic fluid under fluctuating pressure. End plugs are often tasked with a secondary role functioning as a spring stop or valve stop.
The repetitious and varying actions of fluid pressure, spring motion and valve impact cause wear at the end plug and bore mating surface and can distort the shape of the end plug. High-pressure fluid leaking through the clearance gap can, with time, erode the surface material of both end plug and bore. Eventually the cumulative loss of hydraulic integrity leads to low pressure in critical circuits. Complaints vary depending on the location of the worn end plug but are always related to pressure losses.
Sonnax O-ringed end plug kit 168740-10K replaces worn, distorted or eroded end plugs in Toyota/Lexus UA80E/F and UB80E/F series transmissions. Sonnax end plugs have an O-ring seal that reduces leakage further by blocking off worn areas of the bore which the new end plug alone could not seal.
Vacuum testing at the port(s) indicated fails to hold the recommended minimum in-Hg, or if wear is visually detected.