XML-RPC Fault are type of responses. If there was a problem in processing the XML-RPC request,the methodResponse element will contain a fault element instead of a params element. The fault element, like the params element, has only a single value. Instead of containing a response to the request, however, that value indicates that something went wrong. A fault response might look like:
<?xml version="1.0"?>
<methodResponse>
<fault>
<value><string>No such method!</string></value>
</fault>
</methodResponse>
A fault will also have an error code. XML-RPC doesn't standardize error codes at all. You'll need to check the documentation for particular packages to see how they handle faults.