When a minority language holds a subordinate position and is neglected or abandoned in family communication, some speakers acquire it imperfectly, resulting in limited proficiency; these individuals are known as heritage speakers. However, some heritage speakers later develop an attachment to their ancestral language and choose to study it in formal or informal educational settings, becoming heritage learners. This study focuses on heritage learners of Sardinian, a Romance language spoken in Sardinia. Today, most Sardinian residents under fifty primarily speak Italian, but there is a growing interest in Sardinian language courses aimed at reconnecting community members with their linguistic heritage. This research examines the grammatical and lexical competence of a sample of these heritage learners, particularly focusing on areas identified in the literature as vulnerable to erosion and interference from Italian. Specifically, it will demonstrate that in instances of grammatical or lexical divergence between Sardinian and Italian, heritage learners tend to accept both idiomatic Sardinian forms and Italian-influenced forms, indicating significant interference in their proficiency of the minority language. This study aims to help language teachers develop targeted interventions to enhance the competence of heritage learners, differentiating from curricula designed for second language learners of Sardinian when necessary.