Carlos Sainz has broken ranks to reveal that it was a Renault engine failure that meant he could not start Sunday's Belgian GP.
When it became clear the Spaniard would not be taking to the grid, McLaren - set to switch to Mercedes power for 2021 - announced that it was an "exhaust failure".
"You can imagine how unhappy, how frustrated I am," Sainz told the Spanish broadcaster Movistar.
"On my second lap to the grid, I started to smell something - it smelled terrible. I looked in the mirror and the exhaust was smoking a lot, then it started to sound like the exhaust was broken and, indeed, it was broken.
"But we believe the problem was caused by a problem in the engine - in a cylinder was what I was going to say, but they won't let me say it."
Renault's 2020 engine is actually strong, helping the works team to a good result at Spa-Francorchamps.
Sainz, though, is less happy.
"It's strange, because this engine was running very well. It is the second race for this engine but it already seems like it will not last long," he said.
"The bad thing is that it is not the first problem of the season. We are having a season that is not letting us score points and get a rhythm."
