A few months ago, I installed the latest Exchange 2013 Cumulative Update that was available (CU14) on the Exchange cluster. Shortly after everything was brought back online, I noticed that the Content Index state was marked as failed. Trying to re-seed the content index from another database server wasn’t working either. I tried going through the process to completely rebuild the content index (stop the Exchange search services, delete the content index folder, restart the services) and it seemed to work as the crawling process took 12-16+ hours.
Unfortunately, it never did finish. I kept seeing errors in the event log that made it seem like crawling would finish, but there was a final process that continued to fail time and time again. The Exchange search services would crash constantly, restart, and crash once more – each time failing to bring the content index online.
It turns out that this was a known issue with Microsoft Exchange 2013 CU14 (as well as Exchange 2016 CU3). Microsoft has committed to providing the fix in the next CU, but until then there wasn’t really a workaround. The biggest impact this failure had was that searching public folders was a lot more difficult. You could go through the Advanced Find features and do a search that didn’t access the index, but the instant searches would fail to find anything.
CU15 was released in December and resolves this particular issue (among other issues that may have popped up in the last 3 months).
https://blogs.technet.microsoft.com/rmilne/2016/12/14/exchange-2013-cu15-released/
https://support.microsoft.com/en-us/kb/3202691