code: public function enableFPCDebugAction()
      {
          try {
              $this->getService()->setFPCDebug(1);
              $this->getService()->flushCache();
  
              $message = $this->__('FPC debug was enabled');
              $this->getSession()->addSuccess($message);
          } catch (Exception $e) {
              $message = $this->__('FPC debug cannot be enabled: %s', $e->getMessage());
              $this->getSession()->addError($message);
          }
  
          $this->_redirectReferer();
      }
docs: Enables Full Page Cache Debug