Mercedes boss Toto Wolff says his team is being "uber-cautious" when it comes to the reliability of its two cars over the rest of the season.
With four races remaining, Nico Rosberg and Lewis Hamilton are in a straight fight for the title with 33 points separating them ahead of Sunday's U.S. Grand Prix. Hamilton's campaign has been dogged by a series of reliability issues, including an engine failure while leading the Malaysian Grand Prix last month, and on Friday night in Austin, Mercedes mechanics broke their FIA-imposed curfew to change the fuel system on his car.
"We just saw some data we were not sure of, whether it was all right," Wolff explained. "Our main aim is to give them a reliable car so they can fight it out on track, and we are being maybe uber-cautious at the moment.
Hamilton said he was trying not to think about the potential problems.
"I didn't know [about the fuel system before qualifying]. I don't know why I didn't know -- I was just focused on the job. I wasn't actually aware so I didn't have any concerns at that point but I am conscious of it. Maybe you should ask Toto about it and why there's a knock-on effect."
When Wolff was asked if there could be any ramifications to the fuel system change, he said: "Generally this season we have more mechanical reliability situations which we need to get on top of, and we are trying to contain that at the moment."
Following the engine failure in Malaysia, Mercedes decided to delay the introduction of new engines for its customer teams while it investigated the issue. Despite its customers fitting new engines this weekend as well as Rosberg taking a fresh power unit, Wolff admitted the big-end bearing failure at Sepang is still not fully understood.
"No it hasn't been fully understood yet. It's still under investigation. We've seen this season that sometimes the complexity of the kit makes it difficult to pinpoint down one single thing."