First, be sure you are using the 1.0 version of this check from: https://code.google.com/p/check-yum/downloads/detail?name=check_yum_1.0.0&can=2&q= The changes I've seen in that version could fix this issue, depending on it's root cause.
Second, if your server(s) are not configured to use all 'local' cache repos, then this check will likely time out before the 30 second deadline. Because: 1> the amount of data from the refresh/update is pretty large and may be taking a long time to download from remote (include RH proper) servers and 2> most of the 'official' update servers tend to go off-line A LOT.
Best solution I've found is to have a cronjob to perform your update check at a set interval (I use weekly) and create a log file containing those security patches the system(s) require. Then use a Nagios check, via a simple shell script, to see if said file has any new items in it.