  We consider the potential for line intensity mapping (LIM) of the rotational
CO(1-0), CO(2-1) and CO(3-2) transitions to detect deviations from General
Relativity from $0 < z < 3$ within the framework of a very general class of
modified gravity models, called Horndeski theories. Our forecast assumes a
multi-tracer analysis separately obtaining information from the matter power
spectrum and the first two multipoles of the redshift space distortion power
spectrum. To achieve $\pm 0.1$ level constraints on the slope of the kinetic
gravity braiding and Planck mass evolution parameters, a mm-wave LIM experiment
would need to accumulate $\approx 10^8-10^9$ spectrometer hours, feasible with
instruments that could be deployed in the 2030s. Such a measurement would
constrain large portions of the remaining parameter space available to
Scalar-Tensor modified gravity theories. Our modeling code is publicly
available.
