Mercedes team principal Toto Wolff has conceded the manufacturer's engines have "no durability" after Valtteri Bottas was forced into another change ahead of the United States Grand Prix.
Bottas will serve a five-place grid penalty for taking a sixth internal combustion engine [ICE] of the season after previously receiving back-of-the-grid penalties in Italy and Russia.
"We've unfortunately had since mid-year some reliability issues that keep coming back," Wolff told Sky Sports F1.
"I think we understand now much better what it is but it means there is no durability in running them and that means a potential DNF risk.
"The penalties [for Bottas] are less than with the previous ones but it is still not nice because it is such a tight fought battle.
"If you are going back five [places], even if you are on pole, you are going to have a different life to come back."
Asked if Mercedes was experimenting with new pieces in the engines of other drivers to unlock a fix and "go for gold" on another engine with Lewis Hamilton, Wolff denied this was the case.
"No," insisted Wolff. "You need to have the same specification for everybody so you can't make any difference between the one engine or the other."
Addressing speculation the problem was to do with air valves deep in the engine, Wolff added: "You are being fed with a lot of bull***t.
"We don't need to talk about air valves or anything like this because you and I wouldn't understand what it is."