How To Make Temperature (Celsius) Conversion Program

I'm new to visual basic and have decided to make a temperature conversion program. I've attached a screenshot to aid in my description. The program reads the temperatures on the left(from text boxes), converts them, and displays the answers on the right in text boxes. The program has a fever check at the bottom which displays whether or not the converted values are a fever.

I've managed to get this to work no problem, but I'm trying to get it so if the user enters the temperatures directly into the "result" textboxes, that the fever check still works. Unfortunately I'm having no luck, the fever check will only display a message if the values have been converted, not directly entered by the user. Here is the code I tried(fever.Text is what is displayed in a textbox above the fever check button):

Private Sub FeverCheck_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles FeverCheck.Click
'Turn string values entered into Decimal Values so they can be compared to decimal values
Dim fahrfever As Decimal
Dim Celsfever As Decimal
'The following two lines have problems when debugging
[Code] .....