McLaren has strongly rejected suggestions that its new 2018 car might have fundamental issues with its design.
The team has been struggling with reliability issues in pre-season testing at the Circuit de Barcelona-Catalunya.
Last week the car suffered two different incidents of the bodywork overheating. One was ascribed to a faulty exhaust clip, and the other to a hole in the heat shield which was quickly fixed.
But this week's session saw the McLaren break down and trigger two red flags on Tuesday. Once was due to an electrical shutdown, and the other to a hydraulics issue.
The electrical issue has been blamed by engine suppliers Renault on a "bad batch" of batteries. Red Bull also had a similar issue on Tuesday.
"We have found some that are not fit for purpose," said technical chief Bob Bell. "Of course it's an issue for us. But the reason we do this testing of those components before going to Melbourne is to find the ones that have got problems."
However, McLaren's woes continued on Wednesday when an oil leak forced the team to change the Renault engine in Fernando Alonso's car.
"We have some little issues which we have to fix, but it is just testing," insisted sporting director Eric Boullier. "It is part of the process ... We are on top of this."
The team certainly seems in a better position that it was last year, when it suffered a disastrous pre-season test with former engine partners Honda. But Boullier admitted that the switch of suppliers over the winter came with its own challenges.
"It is a new partnership with Renault. New packaging, completely new packaging for the car as well. This is testing, give us time – it is fine."
And he firmly rejected suggestions that the car's tight design left insufficient space to allow the power unit to function properly.
"I don't think it is related, no," he said. "Obviously we try to design the best car.
"With a new car concept you have to prove out on track if it works or not," he continued. "We are trying to make the best out of our car to make sure the aerodynamic platform is delivering much more.
"We are now about to fine-tune, and [today's oil] leak we have is nothing related to that."
Boullier's confidence was not entirely shared by the team's other driver, Stoffel Vandoorne.
"It was a very difficult day for us," he told Belgian broadcaster RTBF.
"We had a lot of technical problems, and now we have to find solutions. We have only three days of testing left and we need this preparation for Melbourne."