Ongoing shortages of science, technology, engineering, and mathematics (STEM) teachers have prompted policy aimed at recruiting career change teachers as a solution. However, little is known about what deters career changers from becoming STEM teachers. This gap is explored through interviews with nine career changers who contemplated, but decided against a career change to STEM teaching. Inductive thematic analysis generated themes and subthemes which were then deductively categorised using Margaret Archer's theories on emergent properties. Findings reveal that career changers are constrained from choosing STEM teaching when they perceive student behaviour as poor, the scope of teachers' work as excessive, barriers to attaining a teaching qualification, or that the profession is not socially valued. Recommendations are presented to reduce barriers for potential STEM career change teachers.