The following code is not working properly...it works for the first cell but then it quits even though there is value in the adjoining cell

Sub Downtime()
Range("G1").Select
For q = 0 To 500
ActiveCell.Value = ActiveCell.Offset(0, 1).Value - ActiveCell.Offset(0, 2).Value
ActiveCell.Offset(1, 0).Select
If ActiveCell.Offset(0, 1).Value = "" Then
Exit For
End If
Next
End Sub