The 2nd gear lubrication while the automatic transmission lock-up is engaged is not sufficient, and therefore, if the lock-up in 5th gear is maintained for certain time, insufficient lubrication can cause the 2nd gear teeth to overheat, which results in a loss of hardness of the gear. Prolonged operation under these conditions can result in broken gear, and in the worst case, a broken piece could be caught between other gear teeth causing the vehicle to stall.
