Decarboxylates ethylmalonyl-CoA, a potentially toxic metabolite, to form butyryl-CoA, suggesting it might be involved in metabolite proofreading (PubMed:22016388). Acts preferentially on (S)- ethylmalonyl-CoA but also has some activity on the (R)-isomer (By similarity). Also has methylmalonyl-CoA decarboxylase activity at lower level (By similarity). {ECO:0000250|UniProtKB:Q9D9V3, ECO:0000269|PubMed:22016388}. This is the function of ECHDC1 (ethylmalonyl-CoA decarboxylase 1, Ensembl gene identifier ENSG00000093144).