Miguel Angel Morenatti/Associated Press
At the first test in Jerez, the Renault-engined teams had a rotten time. Caterham did 76 laps, but seemed to be running with everything turned right down and were painfully slow.
Toro Rosso did 54 laps, while world champions Red Bull did just 21.
There were many problems with the new Renault Energy F1 powertrain at the last test, but the major issue related to the cooling. This is proving especially troublesome for Red Bull.
The team's chief technical officer, Adrian Newey, told Autosport about the difficulties:
It was, you could argue, a result of aggressive packaging but we felt that we needed to take a few risks to try to get a good package that would minimise the aerodynamic damage of this very large cooling requirement.
The Renault seems to have a particularly large cooling requirement.
Everybody of the three engine manufacturers will have a different target for how hot their charge air is going back into the plenum and Renault have given us a fairly challenging target.
It has all sorts of advantages if we can get there, but it is not easy to achieve.
Renault have been working hard on a solution and the teams will have spent the past few weeks toiling away at finding a fix too. They absolutely need to have everything sorted by the end of this test.
If not, four teams and one engine supplier will have a serious problem.