McLaren-Honda suffered another setback in their preparations for the new season at the start of the final test.
The team were forced to abandon testing after just seven laps because of a hydraulic leak in the Honda engine.
McLaren's car has done only 203 laps, none of them at close to full racing speed, in nine days of testing. Only three days of testing remain before the first race.
Fastest time in Barcelona was set by Williams's Felipe Massa.
The Brazilian headed Sauber's Marcus Ericsson, who was 0.776secs slower on the 'super-soft' tyre that is about 0.7secs faster than the soft used by Massa.
Mercedes driver Lewis Hamilton was third fastest, 1.381 seconds behind Massa's time, but the world champion set his lap on the medium tyre, which is about 1.2secs slower than the soft.
Mercedes also hit trouble, with a problem in the car's hybrid system ending Hamilton's day after just 48 laps - less than half the mileage the team have been doing on most days this winter.
Massa, meanwhile, managed more than 100 on the first occasion Williams came close to revealing something like the pace that many had suspected was in the car.
They will start the season in Melbourne, Australia, in two weeks' time with expectations that they could be Mercedes' closest challenger, just as they ended last year.
But nothing about the day changed the impression that Mercedes remain the team to beat, with a comfortable advantage over the rest.
Hamilton set his time on a run of about 12 laps, while Massa's was on a single lap run, which means the world champion could have gone even quicker had he had less time in the car.
The amount of fuel in the cars is not known, which makes headline lap times in testing an unreliable guide to actual form.
Red Bull's Daniil Kvyat was fourth fastest, ahead of Lotus driver Romain Grosjean and the Ferrari of Kimi Raikkonen.
The test ends on Sunday and the cars are freighted to Australia at the end of next week. First practice starts at 12:30 local time (01:30 GMT) on Friday 13 March with the race on 15 March.
Final pre-season test. Day one, Circuit de Catalunya
1 Felipe Massa (Brz) Williams-Mercedes 1:23.500 (103 laps)
2 Marcus Ericsson (Swe) Sauber-Ferrari 1:24.276 (122)
3 Lewis Hamilton (GB) Mercedes 1:24.881 (48)
4 Daniil Kvyat (Rus) Red Bull-Renault 1:25.947 (75)
5 Romain Grosjean (Fra) Lotus-Mercedes 1:26.177 (75)
6 Kimi Raikkonen (Fin) Ferrari 1:26.327 (80)
7 Carlos Sainz (Spa) Toro Rosso-Renault 1:26.962 (86)
8 Jenson Button (GB) McLaren-Honda 1:31.479 (7)