Detecting Problems With ISERROR

Sometimes you do not want to replace an error, you want to ask whether one exists and then decide what to do.

The ISERROR function does exactly that. It looks at a cell or formula and returns TRUE if the result is an error and FALSE if it is fine.

This lets you build your own custom handling, count errors, or flag problem rows. In this lesson you will learn ISERROR and its relatives.

ISERROR takes a single argument, the value or formula to test:

=ISERROR(value)

It returns the logical value TRUE when the value is any error, and FALSE otherwise. It never shows the error itself, only a clean true or false answer.

On its own that is just information. Its real power comes when you pair it with IF.

Combine ISERROR with IF to build fully custom handling. The IF tests for an error, then chooses what to show in each case.

Here, if the division errors, the formula returns "Check data". If not, it returns the real result. This is the manual equivalent of IFERROR, but you control both branches.

The pattern is =IF(ISERROR(formula), do_this, else_show_formula).

ISERROR has close relatives that test for specific errors:

• ISERR is TRUE for any error except #N/A.
• ISNA is TRUE only for the #N/A error.
• ISERROR is TRUE for all errors including #N/A.

Choosing the right one lets you respond to exactly the situation you care about, much like the difference between IFERROR and IFNA.

When you only care about missing lookups, ISNA is the precise test. It returns TRUE only for #N/A.

This recreates IFNA behavior in older spreadsheets, or lets you take a custom action when a lookup misses, such as writing to a different column.

Here the formula reports whether a customer was found at all.

ISERROR is useful for auditing. To count how many cells in a column contain errors, you can combine it with SUMPRODUCT.

Because ISERROR returns TRUE or FALSE, the double negative -- turns those into 1s and 0s, and SUMPRODUCT adds them up.

The result tells you how many error cells exist, a quick health check for a large data set.

You can use ISERROR to add a clear warning column next to your data. Each row checks its own calculation and posts a flag if something is wrong.

Here, any row whose total errors shows a warning symbol, while clean rows stay blank. Scanning the flag column instantly reveals trouble spots.

This is far friendlier than hunting through scattered #DIV/0! codes by eye.

It helps to see how these relate:

• IFERROR(x, y) is a shortcut that runs x once and returns y on error.
• IF(ISERROR(x), y, x) does the same but evaluates x twice, which can be slower on heavy formulas.

So for simple replacement, IFERROR is usually better. Use ISERROR when you need to test rather than replace, or take a different action than just substituting a value.

ISERROR also powers conditional formatting. You can write a rule that highlights any cell whose formula returns an error.

Set the rule formula to =ISERROR(A1) applied across your range, and color matching cells red.

Now errors light up visually the moment they appear, without changing the cell values at all. It is a clean, non-destructive way to monitor a sheet.

Because ISERROR returns a clean TRUE or FALSE, you can use it to route problem values somewhere useful. Pair it with IF to copy only the failing keys into a review column.

Here, if the lookup for a key errors, the formula writes the key itself into the cell; otherwise it leaves the cell blank.

Filtering that column then gives you a ready-made list of every key that needs fixing, turning error handling into an action plan.

Here is the full toolkit for error handling:

• IFERROR replace any error with a fallback.
• IFNA replace only #N/A from lookups.
• ISERROR / ISNA / ISERR test for errors to drive custom logic, counts, or formatting.

Pick replacement functions when you just want a clean display, and the IS-family when you need to decide or measure based on errors.

Test your understanding of the error-testing functions.

You completed the error-handling toolkit:

• =ISERROR(value) returns TRUE for any error, FALSE otherwise.
• ISNA tests only for #N/A; ISERR tests for all errors except #N/A.
• Pair with IF for custom handling, with SUMPRODUCT to count errors, or with conditional formatting to highlight them.
• Use IFERROR or IFNA to simply replace; use the IS-family to test and decide.

Your spreadsheets can now stay clean, accurate, and professional.