We present results from a study of the core‐mantle boundary (CMB) underneath Mexico, Central America, and the northeast Pacific using core‐reflected phases PcP and ScP, which provide a direct means of sampling the deep mantle. Precursor arrivals to these phases produced by reflection from the top of an ultra‐low velocity layer (ULVZ) at the base of the mantle would contain information about the thickness of such a layer and the velocity reductions within it. Stacks of these phases from 17 earthquakes recorded by the Northern and Southern California Seismographic Networks do not exhibit coherent or incoherent precursor arrivals from a ULVZ. In precursor time windows, ratios of stack amplitudes to those of the CMB‐reflected phases fall below predicted values from published ULVZ models. We conclude that ULVZs, if present in these regions, have VP and VS reductions less than 10%, are less than five km thick, or have transitions greater than five km wide at their upper boundaries.