Ferrari have not been forced to replace the entire engine to fix the problem and were able to just change one part ahead of the race in Shanghai.
The Prancing Horse believe the problem that cost Leclerc his first Formula One victory was caused by a short circuit within an injection system control unit.
And the team admitted the issue "had never been seen before” on the part in question.
The work done on both cars appeared to have not hampered its performance as Vettel set the fastest time in the opening practice session of the weekend, with Leclerc managing to come in third.
Vettel knows he must put in a good performance this weekend after failing to make the podium in the opening two races of the F1 calendar.
In Melbourne, Australia the four-time F1 world champion finished 4th after starting third on the grid and things didn’t improve two weeks later.
Vettel qualified second in Bahrain but finished the race in fifth after he spun out and damaged his own car following an on-track overtake by title rival Lewis Hamilton.
However, he will take some comfort from his display in Shanghai as he finished 0.207 seconds clear of Hamilton in the first running.
After struggling for pace in Australia, Ferrari dramatically improved the performance of their car for Bahrain and this appears to have carried through to China.
The good news for Ferrari is that the changes that have been made to the car will not result in any grid penalty.
Championship leader Valtteri Bottas could only manage fifth fastest in the first practice session.
Red Bull's Max Verstappen came in fourth and will be pleased to be within half a second of Vettel's best time.
Vettel admitted his performance his Bahrain wasn’t good enough but is content there still plenty of races to make up for his slow start to the season.
"In Bahrain I wasn't happy with my performance,” he said before the practice session.
“Not happy with the feeling with the car, the feeling that I had prior to the season in testing and so on.
"I know I can do better and I know we have lots of races to show and prove that.”
