diff --git a/plugins/out_s3/s3.c b/plugins/out_s3/s3.c
index 65321112759..99366017f84 100644
--- a/plugins/out_s3/s3.c
+++ b/plugins/out_s3/s3.c
@@ -506,7 +506,6 @@ static int cb_s3_init(struct flb_output_instance *ins,
 {
     int ret;
     flb_sds_t tmp_sds;
-    int async_flags;
     char *role_arn = NULL;
     char *session_name;
     const char *tmp;
@@ -914,8 +913,12 @@ static int cb_s3_init(struct flb_output_instance *ins,
         ctx->timer_ms = UPLOAD_TIMER_MIN_WAIT;
     }
 
-    /* init must use sync mode */
-    async_flags = ctx->s3_client->upstream->flags;
+    /* 
+     * S3 must ALWAYS use sync mode 
+     * In the timer thread we do a mk_list_foreach_safe on the queue of uplaods and chunks
+     * Iterating over those lists is not concurrent safe. If a flush call ran at the same time
+     * And deleted an item from the list, this could cause a crash/corruption. 
+     */
     ctx->s3_client->upstream->flags &= ~(FLB_IO_ASYNC);
 
     /* clean up any old buffers found on startup */
@@ -950,17 +953,6 @@ static int cb_s3_init(struct flb_output_instance *ins,
          cb_s3_upload(config, ctx);
     }
 
-    if (ctx->use_put_object == FLB_TRUE) {
-        /*
-         * Run S3 in async mode.
-         * Multipart uploads don't work with async mode right now in high throughput
-         * cases. Its not clear why. Realistically, the performance of sync mode
-         * will be sufficient for most users, and long term we can do the work
-         * to enable async if needed.
-         */
-        ctx->s3_client->upstream->flags = async_flags;
-    }
-
     /* this is done last since in the previous block we make calls to AWS */
     ctx->provider->provider_vtable->upstream_set(ctx->provider, ctx->ins);
 
@@ -1691,7 +1683,6 @@ static int buffer_chunk(void *out_context, struct s3_file *upload_file,
 static void s3_upload_queue(struct flb_config *config, void *out_context)
 {
     int ret;
-    int async_flags;
     time_t now;
     struct upload_queue *upload_contents;
     struct flb_s3 *ctx = out_context;
@@ -1707,12 +1698,6 @@ static void s3_upload_queue(struct flb_config *config, void *out_context)
         cb_s3_upload(config, out_context);
     }
 
-    /* upload timer must use sync mode */
-    if (ctx->use_put_object == FLB_TRUE) {
-        async_flags = ctx->s3_client->upstream->flags;
-        ctx->s3_client->upstream->flags &= ~(FLB_IO_ASYNC);
-    }
-
     /* Iterate through each file in upload queue */
     mk_list_foreach_safe(head, tmp, &ctx->upload_queue) {
         upload_contents = mk_list_entry(head, struct upload_queue, _head);
@@ -1762,10 +1747,7 @@ static void s3_upload_queue(struct flb_config *config, void *out_context)
     }
 
 exit:
-    /* re-enable async mode */
-    if (ctx->use_put_object == FLB_TRUE) {
-        ctx->s3_client->upstream->flags = async_flags;
-    }
+    return;
 }
 
 static void cb_s3_upload(struct flb_config *config, void *data)
@@ -1781,16 +1763,9 @@ static void cb_s3_upload(struct flb_config *config, void *data)
     int complete;
     int ret;
     time_t now;
-    int async_flags;
 
     flb_plg_debug(ctx->ins, "Running upload timer callback (cb_s3_upload)..");
 
-    /* upload timer must use sync mode */
-    if (ctx->use_put_object == FLB_TRUE) {
-        async_flags = ctx->s3_client->upstream->flags;
-        ctx->s3_client->upstream->flags &= ~(FLB_IO_ASYNC);
-    }
-
     now = time(NULL);
 
     /* Check all chunks and see if any have timed out */
@@ -1867,10 +1842,6 @@ static void cb_s3_upload(struct flb_config *config, void *data)
             }
         }
     }
-
-    if (ctx->use_put_object == FLB_TRUE) {
-        ctx->s3_client->upstream->flags = async_flags;
-    }
 }
 
 static flb_sds_t flb_pack_msgpack_extract_log_key(void *out_context, const char *data,
@@ -2275,10 +2246,6 @@ static int cb_s3_exit(void *data, struct flb_config *config)
     }
 
     if (s3_store_has_data(ctx) == FLB_TRUE) {
-        if (ctx->use_put_object == FLB_TRUE) {
-            /* exit must run in sync mode  */
-            ctx->s3_client->upstream->flags &= ~(FLB_IO_ASYNC);
-        }
         flb_plg_info(ctx->ins, "Sending all locally buffered data to S3");
         ret = put_all_chunks(ctx);
         if (ret < 0) {