Q. If my father tells me do something that contradicts the will of my grandfather (his father), who do I listen to?

A. Rema (Y.D. 240: 24) mentions the opinion of Maharik that one is not obligated to honor his grandfather. However his ruling is that he is, but the honor to a father supersedes the grandfather. Torah Lishma and Teshuva Mehava (1: 178) explain that honoring grandparents is only rabbinical obligation while honoring parents is a biblical commandment. Chelkas Yaakov (Y.D. 135) rules that if both his father and grandfather are present and demand something from him, he should heed his grandfather, since both he and his father are obligated to honor the grandfather (See also Responsa Rabi Akiva Eiger (1: 68) for a similar opinion. Sheilas Yavetz however (2: 129) disagrees.

Horav Shlomo Miller’s Shlt”a opinion is that there are many other variants to this question and additional fact information is necessary in order to establish priority. In some instances there may be no obligation to honor either one, as when it conflicts with mitzvos.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a