EGR is an acronym for Exhaust Gas Recirculation system and it is designed to recycle unburned fuel that would otherwise go into the atmosphere. This also cools the combustion of the engine.  Over time, the EGR system becomes clogged with carbon and stops flowing, allowing that unburned fuel back into the atmosphere. When the EGR system fails, this results in increased combustion temperature. This may cause your head gasket to prematurely fail.  Many people believe this is the main cause of the Prius head gasket failure issue.

We fully clean the entire EGR system when the head gasket is replaced. However, if you have over 100k miles and do NOT have a failed head gasket, then this is a good preventative maintenance