Esteban Gutierrez has described the lack of reliability his Sauber-Ferrari Formula 1 car has suffered in recent grands prix as "unacceptable".
The Mexican finished 15th in last Sunday's Belgian GP, after the control electronics unit was changed on his engine.
Prior to that, his C33 had suffered technical issues Gutierrez described as "related to the power unit" in five consecutive sessions stretching back to the previous race in Hungary.
Gutierrez described this situation as "unacceptable", adding: "at Budapest we were in a position to score some points and due to a technical issue we didn't so that's a big loss to us.
"It's been five times in a row from Budapest. We need to make sure these technical issues are solved because it's really limiting us a lot on track time and [this] obviously limits us to extract the most from the car.
"It is not an acceptable situation I have to say. Still, from my side I need to keep the focus and obviously put pressure on the places where we are weak, because we need to make sure these things don't happen."
Sauber boss Monisha Kaltenborn confirmed the various issues were all related to the Ferrari engine, but said each problem was different, making them more difficult to solve.
"It would be easier if it was always one thing, because you would really have to focus on that, but it's always something different," she told AUTOSPORT.
"I fully understand when he says this is unacceptable, because within the limited time you have [on-track] if there is also an issue like this I can fully understand that attitude.
"It's not just driver related, it's not any particular behaviour, and that's something which is very worrying.
"It is worrying to them [Ferrari] as well, because obviously they don't want this to happen."
Ferrari told AUTOSPORT it would work closely with Sauber to get to the bottom of the issues.
A Ferrari spokesman said: "Reliability is a key performance indicator we knew this at the beginning of the year and we knew it would be even more important in the second part of the season.
"We try to support and collaborate as much as possible with our two partner teams, and it's in our interest to resolve the issues they have because there are lessons we can learn to improve the overall competitiveness of our engine."