Williams is carrying a "very strange" problem around its Mercedes Formula 1 engine as its "main concern" into this weekend's British Grand Prix.
The team suffered a double DNF at the previous race in Austria last weekend, where Alex Albon experienced a suspected recurrence of an engine cooling issue that forced him to retire in Canada and also impacted him in Spain before that.
Team-mate Carlos Sainz failed to even make the start after a brake problem meant he could not pull away on the formation lap and then led to a fire when he tried to start from the pitlane - one race after suffering brake cooling issues in Canada.
Though Sainz's problems appear to be separate, he said that Albon's engine issue is the team's primary concern, as it is the same thing each time and only happening in races.
"Obviously, there's a certain level of concern within the team of having so many reliability issues," said Sainz.
"Three in a row with Alex and the brake issue with me. The brake was a lot of things coming together that we understand now why it happened and it shouldn't happen again.
"The issue on Alex's car that has happened a few in a row now is something that obviously we are trying to solve and trying to understand, and for that we are doing everything we can to understand it this weekend.
"It's a very strange issue that only happens on race day. We only see it happening on race day. So you cannot simulate it at certain points of the weekend, even though we try our best."
Sainz called the issue on Albon's side "the only recurring reliability issue", whereas "everything else is one issue at a time", which makes a permanent solution tricky.
"There's little things popping up every weekend," Sainz said. "And normally, those issues come up at the most stressful points of the weekend.
"It might be Q1, might be Q2, or it might be around the race at some point. And that's what, in the end, prevents you from getting the result and from exploiting the maximum capacity of the car.
"It reflects on a low point tally and the lack of results that I wish we could achieve together. That makes it obviously frustrating, because it's one issue at a time, except for the reliability one, that is now our main concern."
After retiring in Austria, Albon said, "I'm a little bit worried [for Silverstone], I don't know what we can do.
"We can't afford it to happen in Silverstone because that's a good track for us."
Albon revealed on Thursday at Silverstone that the team will conduct tests during FP1 to try and diagnose the problem.
"It would be nice to finish the race on Sunday," Albon said at Silverstone.
"We know what area we need to focus on, hopefully FP1 produces some results that we can move on from."
Upgrade hopes
Williams has only scored one point in the last three races, fewer than any other team.
This, combined with other midfield rivals making progress with upgraded cars, has put it under increasing pressure for fifth in the constructors' championship - a position it looked so solid in just a few races ago.
Asked by The Race about the threat there in the context of Williams not upgrading this car as it focused on 2026 and with the competitive momentum now shifting, Sainz revealed there is an upgrade coming that was signed off a long time ago.
"It was developed a long time ago," he said. "Even though it hasn't been developed based on our feedback from testing and the first few races, there's an upgrade in the pipeline that has been there for a while.
"So we are also encouraged by that. We are not just not going to keep falling back for sure.
"And at the same time, even if we haven't developed the car, I'm extremely confident that in both Canada and Austria, even Barcelona, we would have scored a point if we executed things well.
"There are all these consecutive weekends in a row where the car potential and the speed of the team, the two drivers, has been much higher than the results that we've achieved.
"We just know that we just need to do normal weekends like we were doing at the beginning of the season, and it will come."
