Brazilian Grand Prix practice day threw up more than just one issue for Kamui Kobayashi.
The Japanese driver's position in the team was publicly revealed an hour before first practice when Sauber confirmed Esteban Gutierrez for 2013.
Kobayashi then struggled with the car in the ensuing practice sessions.
"We did a lot of set up work today and also tested some aerodynamic items. We had a busy day, but performance wise we are not good yet", he said.
"Because of the high track temperatures the rear tyres overheated. The lap times were slower in the afternoon than in the morning. The rear is really weak and so far I don’t know how to cool the tyres. We will now work on that".
Brazil is also Sergio Perez's final race for Sauber, although the Mexican heads to McLaren for 2013.
"It was a good day in terms of getting done everything that was on the programme. We did a fair amount of work, but the heat just doesn’t suit us", he said.
"I found the car very difficult to drive this afternoon and I think some big changes are required. The difference between the two tyre compounds is really marginal. Let’s see how we can improve".