Mercedes Trackside Engineering Director Andrew Shovlin has revealed the issue that confined George Russell to the garage for half of first practice for the Miami Grand Prix.
The Briton complained of a vibration during his first laps before returning to the garage with an apparent steering issue.
After extensive work on the front end of the W14 and a red flag after Nico Hulkenberg's crash, Russell made up for lost time and set the session's fastest time with his final lap.
Speaking to Sky Sports F1, Shovlin explained: "We had an issue on George's car, a test item we were evaluating wasn't quite as transparent as we wanted so we had to get that off which then cost him a bit of time and then we flipped the programme around a bit.
"So with the timing of that red flag, we decided to look at the long-run work and put the new tyres on at the end. So part of the reason we are up there is we were the car on track, on new tyres and got clean laps.
"But we have had races this year where no matter what we did, we could never get up near the top of the timesheets so I think it is encouraging but one to keep in perspective."
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Mercedes struggled for pace in Azerbaijan last week but has sparked optimism of a more competitive weekend by finishing one-two in FP1.
"Baku, we often struggle with getting that front end to come in and even when we have had really solid cars, we have often found Baku to give us a bit of a headache," said Shovlin.
"A more normal track, we were hoping it would be a bit similar to Melbourne where we went pretty well. We will see how the weekend progresses but the problems to fix here don't seem to be as big as they were in Baku."