  We study vacua of N = 4 half-maximal gauged supergravity in five dimensions
and determine crucial properties of the effective theory around the vacuum. The
main focus is on configurations with exactly two broken supersymmetries, since
they frequently appear in consistent truncations of string theory and
supergravity. Evaluating one-loop corrections to the Chern-Simons terms we find
necessary conditions to ensure that a consistent truncation also gives rise to
a proper effective action of an underlying more fundamental theory. To obtain
concrete examples, we determine the N=4 action of M-theory on six-dimensional
SU(2)-structure manifolds with background fluxes. Calabi-Yau threefolds with
vanishing Euler number are examples of SU(2)-structure manifolds that yield N=2
Minkowski vacua. We find that that one-loop corrections to the Chern-Simons
terms vanish trivially and thus do not impose constraints on identifying
effective theories. This result is traced back to the absence of isometries on
these geometries. Examples with isometries arise from type IIB supergravity on
squashed Sasaki-Einstein manifolds. In this case the one-loop gauge
Chern-Simons terms vanish due to non-trivial cancellations, while the one-loop
gravitational Chern-Simons terms are non-zero.
