sendFault() should only be used for declared fault, so if the content is XML, service interface should be WSDL and the fault should be declared in WSDL. Otherwise it should throw HandlerException.
https://github.com/jboss-switchyard/switchyard/blob/master/components/camel/camel-switchyard/src/main/java/org/switchyard/component/camel/switchyard/SwitchYardConsumer.java#L116