I studied this model and I can see that the coupling equations are created OK and are consistent from the start to finish of each solve command. What happens is that in the second load step the coupled DOF do not have the same AZ solution values. The incremental AZ solution values are coupled but not the total solution values. It turns out that this is expected since in a nonlinear problem coupling cannot properly keep the total AZ solution values together for each coupled set of nodes. Constraint equations can do so with the extra constant term involved with CEs. In a nonlinear analysis, newly introduced CPs (and therefore CPINTF) simply couple the upcoming *incremental* AZ (what we are solving for in a nonlinear analysis using the Newton method K*dU = R-F ) and enforce them to be the same. The *total* AZ is what it was and is not affected by these new CP. With CEs, however, we internally introduce a constant term in the equation equal to the amount that the AZ of the newly constrained AZ’s differ, thereby enforcing the *total* AZ to be the same.Since CPs do not have this concept of a “constant term”, there is no way to do this with them. (Note though that in some applications coupling the DOFs from “here on out” is a desired effect hence we do not block it!).