In this work, we propose an a posteriori goal-oriented error estimator for the harmonic (A − φ) formulation arising in the modeling of eddy current problems, approximated by nonconforming finite element methods. It is based on the resolution of a dual problem associated with the initial one. For each of these two problems, a guaranteed equilibrated estimator is developed using some flux reconstructions. These fluxes also allow to obtain a goal-oriented error estimator that is fully computable and can be split in a principal part and a reminder one. Our theoretical results are illustrated by numerical experiments.