Option Explicit

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Dim Cel As Range
Dim rngC30C187 As Range
Dim counter As Long
Dim C6Check As Boolean 'Default is FALSE at declaration

Application.EnableEvents = False 'in order that no one can change selection while code is running

Set rngC30C187 = Sheets("Data").Range("C30:C187")

Select Case Sheets("Data").Range("C6").Value
    Case "CP18", "CP18 TM", "M21Z", "M25Z"
        C6Check = True
End Select

If C6Check Then
    rngC30C187.Interior.ColorIndex = xlColorIndexNone
    Range("H30:H187").Value = ""
    
    For Each Cel In rngC30C187
        If (Cel > 2.25 Or Cel < -2.25) Then
            Cel.Interior.ColorIndex = 44
            Cel.Offset(0, 5) = "Error " '& counter 'Offsets to column H
        End If
    Next Cel
End If

Application.EnableEvents = True
End Sub
Personally, I prefer to use an Excel Control Toolbox button to run subs like yours. Or sometimes I format a cell to look like a button then use 
Worksheet_BeforeDoubleClick(ByVal Target As Range, Cancel  As Boolean)
If Target address Not "A2" Then 'use your cell address here
   Cancel = True
   Exit Sub
End If
 'regular code goes here