McLaren has changed Carlos Sainz Jr's Formula 1 car chassis in an attempt to resolve the cooling issues that the Spaniard suffered at Silverstone.
Sainz's car was running hotter than that of team-mate Lando Norris over the 70th Anniversary Grand Prix weekend, and the team was forced to open up extra cooling, which created aerodynamic compromises.
On Thursday in Barcelona Sainz admitted that there was still "a question mark" over the source of the problem, although he trusted the team to address it.
Team principal Andreas Seidl has said that as part of its quest for an answer the team had switched chassis around a race earlier than planned.
"We did different changes to the build of the car, simply to try to understand or exclude the temperature difference that we had in Silverstone," Seidl explained. "So we changed several components, including the monocoque as well, because the radiator ducts are connected to the monocoque.
"What we normally have is a rotation of monocoques during the year, as a standard, to simply distribute the mileage evenly and repair the monocoques, small little repairs, and it was planned to do anyway for next weekend in Spa.
"But with the temperature differences we had in Silverstone we put this change forward by one race, and in the end the radiator inlets are part of the monocoque, which could be one reason of many for the temperature difference. In the end we're trying to exclude as many items as possible.
"We need to see today in FP1 and FP2, if we could get to the bottom of the issue that we had. I think it's also important to understand that in terms of pure performance, that doesn't really make a big difference.
"But with the car in Silverstone running 2-3 degrees hotter than the other car, it simply triggered exactly the threshold where we had to open up the bodywork on Carlos's car, which then was a downside on drag, downside on top speed, and then also a deficit in lap time.
"And obviously we need to make sure that we get to the bottom of it, and avoid that in the future."
Seidl conceded that Sainz has endured most of the team's bad luck of late: "If you look back at the first five races, especially on Carlos's side, there were a lot of things happening on the pitstop side, like this temperature topic.
"Or the tyre blow-up in Silverstone, which were out of his control, and which caused him to lose a lot of points compared to Lando - despite Carlos putting in some great performances in qualifying, and also in the races.
"So it's our task to do better here. And hopefully we can start doing that from Barcelona at his home race onwards."