yece.

Headword: 
yece.
Principal English Translation: 

but, new thought or phrase indicator; nevertheless; still; yet; nevertheless

Orthographic Variants: 
hece, yeceh, yeçe
IPAspelling: 
yeːseh
Alonso de Molina: 

yece. empero, omas. (conjunction aduersatiua.)
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 34v. col. 1. Thanks to Joe Campbell for providing the transcription.

Frances Karttunen: 

YĒCEH but, however / empero o mas (M), pero (C) In C this is attested equally often with the vowel of the first syllable marked long and unmarked. Considering C’s tendency to omit vowel-length marking in frequently used words when they are not central to the example, YĒCEH is probably correct.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 336.

Lockhart’s Nahuatl as Written: 

particle. but, however, nevertheless.
James Lockhart, Nahuatl as Written: Lessons in Older Written Nahuatl, with Copious Examples and Texts (Stanford: Stanford University Press and UCLA Latin American Studies, 2001), 242.

Attestations from sources in English: 

Auh yeçe ma huel iuh xicmatican yhuan ma huel yuh ye in amoyolo, ca amo amechmaquixtia in in ixpantzinco in Dios, in anquitoa, ca yehuatl in octli quichihua, ca aocmo nicmati in tlen onicchiuh in tlen onicnotequiuhti? = Yet know when and be certain that it will not save you before God to say "the pulque did it, I no longer know what I did [or] what I was responsible for."
Bartolomé de Alva, A Guide to Confession Large and Small in the Mexican Language, 1634, eds. Barry D. Sell and John Frederick Schwaller, with Lu Ann Homza (Norman: University of Oklahoma Press, 1999), 125.

huel mococohua ynonacayo amo pactica yece yn notlamachiliz cenca pactica = my body is very ill and not healthy, but my understanding is very sound (Coyoacan, 1622)
Beyond the Codices, eds. Arthur J.O. Anderson, Frances Berdan, and James Lockhart (Los Angeles: UCLA Latin American Center, 1976), Doc. 4, 64–65.

yece ynin amo yehuatl mottaz moneltiliz = However, this does not seem to be credible. (central Mexico, early seventeenth century)
Codex Chimalpahin: Society and Politics in Mexico Tenochtitlan, Tlatelolco, Culhuacan, and Other Nahuatl Altepetl in Central Mexico; The Nahuatl and Spanish Annals and Accounts Collected and Recorded by don Domingo de San Antón Muñón Chimalpahin Quauhtlehuanitzin, eds. and transl. Arthur J. O. Anderson and Susan Schroeder (Norman: University of Oklahoma Press, 1997), vol. 1, 152–153.

maçonellihui y ninococohua hece yn noyollia y naniman camo quen catqui ca çan pactica = even though I am ill, nonetheless my spirit and soul are tranquil and healthy (Culhuacan, 1580)
Testaments of Culhuacan (provisionally modified first edition), eds. Sarah Cline and Miguel León-Portilla, online version http://www.history.ucsb.edu/cline/testaments_of_culhuacan.pdf, 8.