It's a listed limitation of TurboTax that TurboTax's Maximize function cannot be relied upon when elective deferrals are also made to another employer's plan.
In this case, since the maximum elective deferral was already made to the plan of the employer that provided the W-2, no elective deferrals can be made to the individual 401(k). Only the profit-sharing contribution can be made. You can get TurboTax to calculate the maximum employer contribution by using the Maximize function for a Keogh or SEP contribution instead of the one for an individual 401(k).
