The weight spectra of the Reed–Muller codes RM(r, m) were unknown for \(r=3,...,m-5\) . In IEEE Trans. Inform. Theory 2024, Carlet determined the weight spectrum of \(RM(m-5,m)\) for \(m\ge 10\) using the Maiorana-McFarland construction, where he also studied the difficulties of trying to extend the results to \(RM(m-6,m)\) . In this paper, we fully determine the weight spectrum of \(RM(m-6,m)\) for \(m\ge 12\) , which gives a positive answer to an open question on the weight spectrum of \(RM(m-c,m)\) for \(c=6\) . Moreover, we put forward a conjecture and verify it for some cases. If the conjecture is true, that open question can be completely solved.