https://phys.org/news/2018-11-grace-fo-resumes.html

Both GRACE-FO satellites are equipped with redundant systems in their MWIs, which are their primary measurement instruments. The MWIs very precisely measure the distance change between the two satellites.

The switchover to the backup system was required after an anomaly occurred in a component of the primary system. The primary unit was powered down on July 19, when an instrument fault monitor detected that it was using less current than expected. After a full investigation by an anomaly response team, the mission team began a series of procedures required to switch over to the new unit. The backup system in the MWI was powered up on Oct. 19.