I know you didn't ask about this and it isn't part of your problem but you might want to think about properly normalizing your table.
You have three columns for types of service:
Sum(IIf([DataT]![Oil & Filiter Change]=Yes,1,0)) AS [Oil Changes],
Sum(IIf([DataT]![General Repair]=Yes,1,0)) AS [General Repairs],
Sum(IIf([DataT]![General Service]=Yes,1,0)) AS [General Services]
When in reality, you should have ONE column which identifies which service was performed. Think about what you would have to change if your boss asked for a new service type.