actions – default post thumbnail and category if there is no thumbnail and category

I want to do following…

1 – Publish the new post,
2 – Check if this new post has a featured image,
3 – If it has not a featured image, define its category and save the specific prepared image as the featured image. = Preparing different images for each categories.

Please have a look at the following code that I am using in my functions.php. but it is not working for me.

add_action('save_post', 'wp_force_featured_image', 20, 2);

 function wp_force_featured_image($post_id, $post) {

if( $post->post_type == 'post' && $post->post_status == 'publish' ) {

if(!isset($_POST('_thumbnail_id'))) {

         $post_categories = get_the_category( $post->ID );
         $cats = $post_categories->term_id;

         if ( $cats = 1 ) {      
          add_post_meta( $post_id, '_thumbnail_id', '3135' );
         }
         elseif  ( $cats  = 2 ) {
          add_post_meta( $post_id, '_thumbnail_id', '3138' );
         }
    }
    }
}