The keyword "keeptmode" means that the index of transition vector will not change. This means that if you specify 2nd mode and "keeptmode" you will in fact not locate the transition structure because you will always maximize along the second vector. The Hessian eigenvectors are always sorted from the lowest to the highest, so at the true transition state the second mode must be positive. This keyword should be used only if you specify the 1st mode and you encounter mode flipping, i.e. the mode oscillates during the optimization.