Using Multiple IF statements within single formula

Hello all,

I'm trying to put together the following formula, however I am getting an error telling me i've entered too many arguments for this function:

=IF(Diesel!A5="Y",IF(Diesel!B5="","",CONCATENATE("BEGIN;ALLOCATION;T;990001619;CG;",Diesel!B5," - ",Diesel!C5,";"),CONCATENATE("MODIFY;ALLOCATION;T;990001619;CG;",Diesel!B5," - ",Diesel!C5,)))

Basically, I need to run an If statement to say:

IF Diesel!A5 = Y THEN perform another if statement.

Is this at all possible or am I barking up the wrong tree?

Thanks guys,

- Jon