The first pre-season test offered a hint of what the season has to offer, but the second is the one where a clearer picture will emerge.
It still won’t be easy to draw definitive conclusions but the realities of what the teams have to achieve over these next four days help those of us trying to work out what’s going on. Race simulations. Qualifying runs. New bodywork - in some cases Melbourne-spec. All help remove some of the variables of the first test, which is focused largely on reliability and learning the cars.
Our first look at Pascal Werhlein in a Sauber car.
Much of the German's opening day is likely to be about acclimatisation as he gets used to the controls, seating position, radio communication with the team etc.
FERRARI GO FASTEST
Vettel sets the new fastest lap of the session, a 1:25.231.
He goes quicker again! A 1.23.298 for the German.
McLaren set their first lap o f the day, and it's a 1:24.346, which puts Vandoorne second.
Vettel also goes quicker again, with a lap time of 1:22.422.
Lance Stroll will be eager to put behind him a less-than-ideal first test after a couple of crashes in his Williams last week.
F1 journalist Dan Knutson in Barcelona: "Lance Stroll garnered plenty of media attention at the first test, and not all of it for the best of reasons as he twice damaged the car and limited the amount of running the Williams team could do. There is a TV crew from Canada for this test, so he will be in the spotlight back home as well.
"Veteran drivers Lewis Hamilton and Felipe Massa have both said the 18-year-old Stroll will need time to settle in. 'I feel for him in the sense that it's the toughest year to come into Formula 1, being in the fastest cars and the most physical cars,' Hamilton said. 'You can't just jump in and drive from no experience at all to being consistent and not spin. That's to be expected. It's actually good for him to go through this now, rather than at the first race.'"
He's got the day off today, with Felipe Massa behind the wheel.
PALMER SETS A LAP
Renault's first lap has been set, with Palmer clocking in a 1:26.446.
The Briton goes quicker again, with a 1:25.466.
He's been joined on track by Massa.
One thing to note about the Mercedes this morning: the shark fin is back on the W08 after the team gave the engine cover design a brief run-out last week.
An encouraging start for McLaren in the opening 40 minutes: the MCL32 is already up to 10 laps with Vandoorne - it took them rather longer to hit that mark last week!
For no-one is this test more critical than McLaren and, in particular, their engine supplier Honda.
The first test, frankly, dismal for them, with reliability problems compromising running and raising the spectre of a fundamental problem with the redesigned Honda 2016 engine.
Honda’s F1 boss Yusuke Hasegawa admitted to being “worried” by the engine failure. Honda did not know what caused it at the time, and have only said since that investigations are ongoing.
McLaren racing director Eric Boullier was not much more encouraging. “Can we get back on track with what we’ve expected? I don’t know. We have to investigate what is wrong.”