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We ask for 6 WFC3 orbits to obtain G141 grism spectra of two Type Ia supernovae (SNe Ia) in order to ascertain why the newly-discovered near-infrared (NIR) year-long plateau at virgul150-500 days comes to an end. SNe Ia have long been used as standard candles to measure extragalactic distances and constrain cosmology comma yet the nature of their progenitor stars and the physics behind their explosions remain open questions. The recent discovery of the NIR plateau highlights just how much we still have to learn about the physics of these explosions. In this proposal comma we aim to learn why the NIR plateau comes to an end at virgul450-500 days past maximum light. The discoverers of the plateau proposed that a change in the dominant ionization state of the SN ejecta comma from singly-ionized to neutral iron-group lines comma may be the cause. However comma this explanation rests on a dubious detection of neutral iron lines in a low signal-to-noise (S-N) grism spectrum of a single SN Ia at 600 days. We will obtain higher S-N spectra of two brighter SNe Ia and optimize the orients of our observations to properly subtract any contamination from the bright host galaxies of our targets. This will allow us to unambiguously detect any CORCHETE_OPENFe ICORCHETE_CLOSE lines in our spectra comma if they exist. By visiting our targets at additional comma earlier epochs along the plateau we will also determine whether such CORCHETE_OPENFe ICORCHETE_CLOSE lines exist along the plateau comma something that could not be done with previous comma ground-based spectra where these lines fell in atmospheric absorption bands. The results of our observations will provide fresh constraints on the chemical composition of the progenitors prior to explosion as well as the evolution of the SN ejecta.