Check engine light goes on, and the cruise control light blinks. The vehicle's owner is concerned.
Drive the car to an auto parts store (O'Reilly, formerly Murray's), borrow their diagnostic tool, read out the code. The tool itself doesn't say too much about what it is, but it was free to use, and it was easy to use. Note to self: next time the check engine light goes on on any vehicle, take it there first.
Spend some time searching online; after what seems like too much time, find this Subaru service bulletin, titled Diagnostic procedures for P0851/P0852. See also a copy of this on the BSR site, also Diagnostic procedures for P0851/P0852.
An answer on JustAnswers makes it look like a routine task and not a safety concern, but with all of this car stuff, it's hard to know precisely.