The C21 was a direct evolution of the 2001 C20 that had carried Sauber to fourth in the Constructors' standings. Rampf and his team retained the overall concept while making targeted changes to the radiator positioning, suspension setup, and rear bodywork. A new gearbox, more compact and lightweight than the previous unit, was also introduced.
The team continued using Ferrari engines badged under the Petronas name, as had been the case since 1997. Throughout 2002 Sauber ran derivatives of the Ferrari 051 engine — the unit that had powered Ferrari's own F2001 chassis — at a supply cost of £13 million. Bridgestone continued as tyre supplier.
Development of the C21 was constrained by the absence of an in-house wind tunnel. The team was in the process of constructing one at their Hinwil base, but it would not become operational until the 2004 season. Wind tunnel work on the C21 ceased entirely in August 2002 as resources shifted to the design of the following year's C22. Despite this, aerodynamic, floor, engine cover, front wing, and weight reduction improvements were made across the season.
Nick Heidfeld returned for his third season with Sauber, paired this time with Felipe Massa, who was making his debut season in Formula One. Massa replaced Kimi Räikkönen, who had moved to McLaren. Heinz-Harald Frentzen deputised for Massa at the United States Grand Prix following a controversial grid penalty issued to the Brazilian driver, and Sauber ultimately chose Frentzen to replace Massa permanently for 2003.
The C21 was competitive in the opening rounds, with Sauber rated as the fifth fastest team behind Ferrari, Williams, McLaren, and Renault. Both drivers were eliminated in a first-corner accident at the Australian Grand Prix, but they recovered with a double points finish at the Malaysian Grand Prix, Massa scoring in only his second grand prix. Further points came at Barcelona, Silverstone, Hockenheim, and the Nürburgring.
Technical failures and driver errors cost the team more points across the season. Heidfeld suffered brake failure at Interlagos and radio problems at Imola, and he collided with Takuma Sato's Jordan under braking at the Austrian Grand Prix. Both drivers were hit with drive-through penalties in Canada after electronic faults caused them to exceed the pit lane speed limit.
Massa's season was marked by several retirements and collisions, most notably at the Italian Grand Prix, where contact with Pedro de la Rosa's Jaguar earned him Formula One's inaugural ten-place grid penalty for the following race. The team's pace fell away markedly in the second half of the season, with Sauber failing to score in the final five grands prix. At the Belgian Grand Prix the drivers could manage only 17th and 18th on the grid.
Sauber finished fifth in the Constructors' Championship with eleven points. The margin over the chasing Jordan, BAR, and Jaguar teams was narrow, and the team acknowledged that fortune played a role in holding their position. Heidfeld retired from just three races all season, providing a backbone of consistency, but the C21's overall competitiveness was a step backward from the C20.
The C21 carried essentially the same livery as the C20: blue from Red Bull, Petronas cyan, and Credit Suisse white, with Red Bull branding repositioned to the airbox. In the final three races of the season — Italy, the United States, and Japan — the team's bargeboards were painted white and carried the slogan "Stop AIDS" in support of an international awareness campaign.