McLaren's Lando Norris was fastest on the opening day of F1's second pre-season test, while rivals Mercedes and Ferrari each ran into problems.
Teenage rookie Norris recorded an encouraging time of 1:17.709 to set the pace in Barcelona ahead of Red Bull's Pierre Gasly, although he also twice had to be recovered from the track.
The 19-year-old's quickest lap came on Pirelli's fastest tyres, the C5 compound, before his MCL34 car stopped for the second time having earlier broken down due to a hydraulics fault.
World champions Mercedes lost most of the afternoon session because of a problem with oil pressure.
Valtteri Bottas, who replaced team-mate Lewis Hamilton at lunch, had his track time severely reduced after his W10 stopped on his first lap out of the pits. After a three-hour delay and an engine change, the Finnish driver returned for an six additional laps.
World champion Hamilton, who finished second last, had earlier competed a trouble-free 83 laps in a car which underwent a host of aerodynamic upgrades after appearing to be lagging behind front-runners Ferrari following last week's first test.
Ferrari's setback came during the opening session of the day when a problem with the cooling systems forced them to remain in the garage for two hours during Charles Leclerc's scheduled time at the wheel.
Two-time world champion Fernando Alonso, who retired from F1 at the end of last season, was unable to stay away from the paddock for too long and was an interested spectator during the afternoon.
Red Bull led for large periods on Tuesday but Gasly was ultimately forced into second place, 0.006s behind Norris, with Racing Point's Lance Stroll third on the timesheet.
Sebastian Vettel finished fourth, with his time of 1:17.9 Ferrari's best of the winter, while Antonio Giovinazzi in the Alfa Romeo and Toro Rosso's Alexander Albon finished ahead of the German's seventh-placed team-mate Leclerc.
Elsewhere, there were improving signs for Williams.
After an embarrassing opening week when their car was not ready for the first two-and-a-half days of running, George Russell completed 119 laps to record their first lap-count century of 2019.
