That's pretty darn hot (> 150ºF). It must be located in a well ventilated area, with nothing blocking the vent ports. Sometimes customers stack items or other devices, which can cause overheating. If this is not the case, you should be able to hear the fan working up close. If the fan is not working, that would cause overheating too.
Try disconnecting the HR24 and letting it cool down for an hour or two. Then plug it back in (may have to wait up to 24hrs for progamming info to load) and see what happens. Keep us posted.
TechKnow Guides and installer/tech experts can also help troubleshoot.