Back in January this year, Google introduced a new version of "Ad Preview Tool" that integrates ad diagnosis: instead of simply showing you a preview of the search results for a given keyword, the tool now also tells you if your ad is showing for that search. If not, you can see the reason why.
Google today announced that they're now retiring the separate Ads Diagnostic Tool and directing you to Preview Tool. "We'll consolidate names too; you'll see a single Ad Preview & Diagnosis tool under your account's Reporting and Tools tab in the future," Google said.
Google also integrated diagnosis options directly into your Campaigns tab. If you've been using Ads Diagnostic Tool option to diagnose many keywords at once, -- you can find it without leaving your Keywords tab! Just click "Diagnose keywords..." under "More actions" menu to get integrated diagnosis results directly on the page:
To interpret the distribution of results more quickly, click "Filter results" link, then "Show diagnostic status" in the filter panel that opens. This'll open additional filter options specific to results returned by a diagnosis. With filters, it's easy to focus on a specific set of issues and then make the necessary changes to get ads back up and running!
More Info: Help Center