Comparing ranges to see if duplicate values occur

Try this:

1. Make a note of the range of the old panels, with the sheet name - in your sample file, this is 'Old Panels'!$A$1:$A$950.
2. Go to the New Panels sheet and select the entire range of the new panels - in your sample file, this is A1:BE51.
3. On the Home tab click Conditional Formatting then New Rule then Use a formula to determine which cells to format.
4. Enter this formula:

Formula:

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If your actual range starts in another cell, not A1, use that cell ref instead.
5. Choose the formatting you want (yellow fill, red text, whatever you want).
6. Click OK.

Now all the cells which have codes which do not exist on the Old Panels list will be highlighted.

(A word of warning - sometimes Excel does something weird and changes the cell reference you enter in CF to some really-far-away cell like XDA1048527. If you do the above and all your cells are highlighted, check the CF rule, using Manage Rules, and it's probably this which has happened - change the weird cell ref to A1 again and click Apply.)

I've attached your sample file with this working - if this is your actual file, then you're in luck and the work's already done .

Hope that helps.

You're welcome, glad I could help and thanks for the rep.