Toro Rosso had to bring its first day of 2016 Formula 1 testing to an early finish at Barcelona on Monday after a gearbox problem developed on the STR11.
Carlos Sainz Jr had a busy morning in the team's plain blue 2016 racer, but he was unable to get beyond 55 laps when Toro Rosso stopped its programme as a precaution in the afternoon.
Chief race engineer Phil Charles said: "We completed our build-up runs in the morning and everything was progressing quiet well.
"Unfortunately, during one of the early runs this afternoon we saw something in the data that's a little bit strange in the gearbox oil system, so as a precaution we decided to stop the car.
"We will now take our time to understand it and hope for an even better day tomorrow.
"Overall, we are pleased with today after such a late decision on our power unit [switching from Renault to Ferrari], so to get to this point and run the car relatively successfully straight away is very pleasing."
Sainz said he was happy with the car until the problems developed.
"It's unfortunate we had to call it a day, but up until then it was all going perfectly and I felt good in the car," said the Spaniard, who ended the day seventh fastest.
"It was a great achievement to drive out after not the easiest of winters.
"It was quite a decent day - we managed to have a very clean morning and it was also being a smooth afternoon until we suddenly had to stop the car."
Sainz hands the car over to team-mate Max Verstappen for the second day of testing on Tuesday.