Pierre Gasly said a suspected issue with the transmission prompted an early end to his day of testing the RB15 in Spain.
Gasly caused the second and last red flag at the Circuit de Catalunya late into the afternoon session. He had become the first driver to rack up 100 laps prior to the stoppage.
The Frenchman came to a halt at the end of the pit lane and the car was taken back to the garage with Gasly finishing P5 on the timesheet.
“It was a busy day in the car and we managed to get through quite lots of test items,” Gasly told the official Red Bull website.
“Today was all about trying to establish a direction for the races to come and also to extract a bit more performance from our package.
“We had what looks like a transmission issue right near the end of the session, which is being looked at now, but we still managed to get a lot of laps done.”
Simon Rennie, group leader of simulator engineering, gave a detailed description of Gasly’s on-track schedule and is expecting Red Bull to be back up and running first thing Wednesday morning.
He said: “There are always things you’d like to try with the car that just aren’t possible on a race weekend, so based on what we learned during the race weekend here in Spain, the plan today was to look at a variety of set-up concepts that we believe will aid general performance across the rest of the season.
“That meant a pretty intense schedule for Pierre and for everyone in the garage as we worked through those changes, but we managed to get a lot done and we acquired some good data.
“In the morning Pierre ran on the C2 compound that was nominated as the medium tyre last weekend. Then, in the afternoon, he spent a short period on the C1 compound for some aero runs before focusing on the C3 tyre up until the stoppage.
“We’re looking into the cause of that now, but we’ll certainly be up and running again for tomorrow’s session.”