Sauber admitted that its lack of performance during the Singapore Grand Prix weekend was due to its latest upgrade package failing to pay off, following a poor qualifying performance.
Two weeks after Sergio Perez's near-victory in the Italian Grand Prix, Sauber has been off the pace throughout the Marina Bay weekend.
Perez will start 14th, while his team-mate Kamui Kobayashi - who had expected a good weekend - could not make it beyond Q1 and starts 18th.
"We have obviously had a difficult weekend here in Singapore," said Perez.
"The main issue is that for some reason we didn't get our new package to work. This comes as a surprise because so far all the development steps have paid off. There is a lot of downforce missing.
"My last lap in Q2 was clear and I think I got the most out of it. We have tried a lot to improve the car's set-up and everything else, but it didn't work out."
Kobayashi added that he had "no confidence" in the car.
"We have tried everything but could not get rid of the huge oversteer," he said. "I have no confidence in the car and this is really the last thing you want to have to cope with on a street circuit.
"The problem I have here reminds me somehow of the one I had at the Hungaroring, where I also struggled with the car. (http://www.autosport.com/news/report.php/id/101579)
"Given how the car is, I don't see how I can recover in the race from where I am starting, but I will not give up, this is certain."
Head of track engineering Giampaolo Dall'Ara said Sauber now understood why it had gone backwards, but that it could not be rectified on-site.
"The qualifying told the truth of where we stand," he said. "On the positive side we were able to identify where the problem comes from. Nevertheless we cannot solve it here."