Lewis Hamilton is running a new MGU-H design on his Mercedes Formula 1 car at the Spanish Grand Prix following the failures he suffered in Russia and China.
The reigning world champion encountered an identical MGU-H problem in Sochi qualifying to the one that affected him at Shanghai.
Following investigations at the factory, the team decided to run with a new design but given the tight turnaround it was only able to have one ready for Barcelona.
As a result, only Hamilton will run the updated MGU-H, which will be the fourth of his allocation of five before penalties, with Rosberg retaining the MGU-H he has used all season.
Hamilton has taken a new turbo as well, which is also his fourth of the season and means the Briton is likely to take penalties at some point later this year.
Rosberg has fitted a new internal combustion engine and MGU-K, both his second of the season, but he retains the previous turbo so has still only used one in 2016.
Sauber and Haas are running the updated Ferrari spec engine, so their respective drivers have taken new ICEs, turbos, MGU-Hs.
All have also fitted new MGU-Ks apart from Sauber's Marcus Ericsson.
Toro Rosso's Daniil Kvyat and Carlos Sainz Jr, who are running the 2015-spec Ferrari engine, have taken their second ICEs, MGU-Hs, MGU-Ks and turbos, as has the Mercedes-powered Manor of Rio Haryanto.