Honda has admitted that it has not yet hit the reliability targets it wants to reach before the season-opening Australian Grand Prix.
The Japanese manufacturer has completed some major design changes to its power unit following the power unit and energy deployment issues that it faced throughout 2015.
Although there has been some progress in closing down the power deficit, early testing suggests that there is still more improvement needed before it can consider becoming a threat to Mercedes.
Allied to the power situation, Honda has been surrounded by recently speculation that its new engine has reliability doubts too, something that Japanese manufacturer played down ahead of this week's test.
However, Jenson Button's running on Wednesday was interrupted by an hydraulic leak, which caused a small fire.
F1's new project leader Yusuke Hasegawa said after Button's problem that reliability remained one of the big areas that needed to be improved – and that some parts of its power unit were not yet completely sorted.
"In general we checked our reliability on the dyno, so we have already confirmed our engine cell is running on the dyno now for 4000km," he explained. "But of course we have to check in the car as well.
"Reliability is one of the very important things to update from last year to this year. But as I mentioned, still we have some minor problems for the parts, so we have to confirm every part is alright for the race."
When asked, having completed a decent amount of mileage so far this week, if he was satisfied, Hasegawa said: "From an operational point of view, definitely. Yesterday we could run over 100 laps. But until 4000km...this is not long enough."
Honda is expected to bring a new specification power unit to Barcelona for the second test as it closes in on the version it will homologate and use from Melbourne.
When asked by Motorsport.com how much of the gap to Mercedes had been closed, Hasegawa said: "We don't know the current position. But still I think they have an advantage and we need to keep up more effort, I think."