Tax query not producing any results

The term parameter should actually be terms, even when providing just one term:

$args = array( 'post_type' => 'course', 
               'tax_query' => array(     
                    array(         
                        'taxonomy' => 'course_codes',
                        'field' => 'slug',         
                        'terms' => 'h001'         
                  )), 
                 'post_status' => 'publish' 
          );