Protein required for the cotranslational protein quality control in the inner membrane of the mitochondria (PubMed:38199007). Associates with newly synthesized polypeptides and may act as a chaperone that cooperates with OXA1L for the insertion of newly synthesized mitochondrial proteins into the inner membrane (PubMed:38199007). Required for the assembly of the ND4 module of mitochondrial complex I (PubMed:33879611, PubMed:33882309). {ECO:0000269|PubMed:33879611, ECO:0000269|PubMed:33882309, ECO:0000269|PubMed:38199007}. This is the function of TMEM126A (transmembrane protein 126A, ENSG00000171202).