Could you post the entire formula you're now trying to use.

It should be:

=IF(Diesel!A53="X", IF(Diesel!B53 = "","",CONCATENATE("ADD;HIERPRODSUBREFRESH;",Diesel!C53," - DIESEL;WEEKLY;",TEXT(Diesel!$B$2,"yyyymmdd hhmm"),";",Diesel!J54,"")), IF(Diesel!B53 = "","",CONCATENATE("MODIFY;HIERPRODSUBREFRESH;",Diesel!C53," - DIESEL;WEEKLY;",TEXT(Diesel!$B$2,"yyyymmdd hhmm"),";",Diesel!J54,"")))