Carlos Sainz Jr was happy to make progress in the STR12 on Wednesday after racking up 92 laps at the Circuit de Catalunya.
Like McLaren-Honda, Toro Rosso are another team playing catch-up after a challenge first test last week but the Spaniard believes things are starting to look up for Red Bull's junior team.
"I think today was a much better day compared to last week and we've definitely made a step forward in terms of reliability," Sainz told reporters.
"Our test programme this week is a very tight one as we still have a lot of catch-up to do, so it's therefore still too early to make any conclusions.
"Even though we lost a bit of valuable track time, I'm happy with the fact that I was able to do so many laps, because that's what testing is all about!"