2004 Vauxhall Vectra unburnt fuel

have put this thread on last year intermittent problem which l think is neat fuel getting into the exhaust as it smells like diesel and white smoke comes from under the car think it may be a faulty injector not shutting off is there any way to check the injectors to see if one is faulty had it scanned at the local garage no codes stored wonder if vauxhall have this sort of equipment the fault is not there all the time so l suppose it will be difficult to trace any help would be appreciated

Zanderman