The documentation of test takers' achievements has been accomplished through large-scale assessments to find general information about students' language ability. To remove subjectivity, Cognitive Diagnostic Assessment (CDA) has recently played a crucial role in perceiving candidates' latent attribute patterns to find multi-diagnostic information rather than single proficiency classification. However, there are some gaps in the literature about in detail investigation of test takers' listening comprehension language ability in responding to placement test items of a public English language center. The present study aims to validate an English placement test at a language center through a retrofitting process. In an exploratory mixed-method design, 449 participants from the same language center, including 274 females and 175 males, were selected. The performance of randomly selected participants in a language center placement test was analyzed by applying the GDINA model from R-studio packages, to detect Differential Item Functioning (DIF). Results of the study revealed DIF in some items since there is some bias in test items. The implication of this study is to provide meaningful interpretations of respondents' attributes and improve teaching and learning by finding the strengths and weaknesses of candidates. For this purpose, the findings derived from the result of the study can raise the awareness of test developers in preparing unbiased items for the placement test, and at the same time, assist test-takers to become more critical of their English language achievements. It is also helpful for materials developers to become aware of developing materials free from bias.