Kamui Kobayashi enjoyed a productive first day at Barcelona as he completed a day without any "noteworthy problems."
The Japanese driver, who watched the sidelines as his team-mate Sergio Perez put the C31 through its paces on the first two days, had his opportunity to develop the team's 2012 challenger on Thursday.
Kobayashi completed 99 laps with a best time of 1:23.582, which put him third quickest, just over a second slower than pace-setter Pastor Maldonado.
"It was my first day of driving the new car here in Barcelona, and it was a good one," said the 25-year-old.
"We had no noteworthy problems all day and went through a lot of test items. I can feel we have made some progress compared to the test in Jerez.
"But, of course, we have a lot more work to do and there is room for improvement."
Sauber's head of track engineering Giampaolo Dall'Ara added: "We have had a straightforward test day. Kamui did almost a hundred laps.
"His programme was mainly about improving and confirming what we understand about the car.
"For example we were looking for improvements in the front tyre warm-up, evaluating different aero parts and ran different suspension configurations."
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