We study the effective theory of decoupling of a charm quark at low energies. We do this by simulating a model, QCD with two mass-degenerate charm quarks. At leading order the effective theory is a pure gauge theory. We compare the mass dependence of hadronic scales to the prediction based on perturbative matching of the gauge couplings. By computing ratios of hadronic scales we have direct access to the power corrections in the effective theory. We verify that these corrections follow the expected leading behavior, which is quadratic in the inverse charm quark mass.