Here's how to trace the source of errors using the Error Checking tool in Excel:

1. Select the cell containing the error: Identify the cell with the error you want to investigate. It will likely have a red triangle in the top left corner and may display an error code.

2. Go to the "Formulas" tab: Navigate to the "Formulas" tab on the Excel ribbon.

3. Find the "Error Checking" dropdown: In the "Formula Auditing" group on the "Formulas" tab, locate the "Error Checking" dropdown menu.

4. Select "Trace Errors": Click on the arrow next to "Error Checking" and choose "Trace Errors" from the dropdown options.

Tracing the Error:

- Once you select "Trace Errors," Excel will analyze the formula in the selected cell and potentially highlight cells with red arrows.
- These red arrows indicate that the error in the current cell might be originating from the value or formula present in the cell pointed to by the arrow.
- Follow the chain of red arrows to identify the root cause of the error. The cell with the very first red arrow pointing towards it is likely the source of the problem in your formula.

Additional Tips:

- You can click the "Remove Arrows" button (within the "Error Checking" dropdown) to clear the red arrows after you've finished tracing the error.
- The "Error Checking" dropdown also offers other options like "Show Calculation Steps" (for a step-by-step evaluation of a formula) and "Show Error Checking Rules" (to manage error checking rules).
- Hovering your mouse over a cell with a red arrow might display a tooltip explaining the specific error detected in that cell.

By using Trace Errors, you can efficiently pinpoint the source of errors within formulas and troubleshoot issues in your Excel spreadsheets.