It was the second day that BMW Sauber were in action on the Spanish track after they, along with Williams and Honda, gave runs to young prospects Marko Asmer, Dani Clos and Mike Conway on Tuesday.
Klien completed 84 laps, clocking a fastest time of one minute 19.537 seconds.
His time was three-tenths quicker than Formula One’s man of the moment, Sebastian Vettel.
None of the other eight drivers managed to get under the 1:20 mark, with Toro Rosso’s Sebastien Buemi - who raced in GP2 this season - third after a lap of 1:20.209.
Nick Heidfeld will replace Klien behind the wheel of the BMW Sauber for the final two days of testing after teammate Robert Kubica was originally expected to test through Friday. Kubica had to withdraw from testing due to an infected tooth, the German team said on Wednesday.
Japanese driver Takuma Sato, who has been without a team since Super Aguri folded earlier this year, will take the wheel for Toro Rosso on Thursday.
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