Returning to Formula One this season with Caterham, the fan favourite finally had his first taste of the CT05 at Jerez on Friday.
Putting in the laps of the Spanish circuit, Kobayashi posted a best time of 1:43.193 on a wet track.
The Japanese driver, though, was not able to put in any laps when the track was at its best due to an engine problem bringing his day to an early end.
"First I want to say thank you to the whole team for making me feel so at home so quickly," he said.
"My first day in the car may have ended earlier than we'd planned after another problem with the Renault engine, but, from where we've been earlier this week, it's very good to have completed 54 laps.
"For me, it was also good to be back in an F1 car and to be working with a team that I'm already feeling very happy with."
The 27-year-old added that he had been making progress throughout the day and had expected to make more during long runs in the afternoon before the issue arose.
"In the morning we started on a very damp track so we were on full wets until about 1030 when we went onto inters as the track was starting to dry.
"We were working through a number of different engine maps and were definitely making progress but just after lunch we had another engine problem which meant we had to stop.
"We were going to try some longer runs in the afternoon, and I'm sure we'd have made more progress, but now we'll go back to the factory to work through all the information from this test and aim to build on our better last day here when we get to Bahrain."