  The black hole candidate EXO 1846-031 underwent an outburst in 2019, after at
least 25 years in quiescence. We observed the system using \textit{NuSTAR} on
August 3rd, 2019. The 3--79 keV spectrum shows strong relativistic reflection
features. Our baseline model gives a nearly maximal black hole spin value of
$a=0.997_{-0.002}^{+0.001}$ ($1\sigma$ statistical errors). This high value
nominally excludes the possibility of the central engine harboring a neutron
star. Using several models, we test the robustness of our measurement to
assumptions about the density of the accretion disk, the nature of the corona,
the choice of disk continuum model, and addition of reflection from the outer
regions of the accretion disk. All tested models agree on a very high black
hole spin value and a high value for the inclination of the inner accretion
disk of $\theta\approx73^\circ$. We discuss the implications of this spin
measurement in the population of stellar mass black holes with known spins,
including LIGO events.
