JEREZ, Spain (AP) Jenson Button is far away from knowing the true potential of the new McLaren car after power unit problems blighted the first two days of pre-season testing.
Button and his new teammate Fernando Alonso managed only six laps each.
By comparison, Mercedes completed 248 laps over two days and Ferrari's Sebastian Vettel clocked 89 laps as he again posted the best time Monday.
McLaren has switched to Honda engines - rekindling the highly successful partnership of the late 1980s and 1990s - but Button expects a tough challenge leading up to the season-opener in Australia on March 15.
''Hopefully we'll get to the first race and we'll be competitive but we don't know where we stand right now,'' said Button, the 2009 F1 champion. ''In terms of understanding our performance, we're still at zero. We don't know where we are.''
McLaren's strong driver lineup promises much - considering Alonso has won two F1 titles - but Button is steering clear of making predictions.
''I haven't even driven the car at full speed (yet) so I don't really know. I could pick a number or position out of a hat but it would mean nothing,'' he said. ''We're not going to be setting the world alight with lap times or laps on the circuit.''
The problems here on the southern Spanish circuit of Jerez follow on from McLaren's difficulties in testing in the two days following the Abu Dhabi GP in late November.
''Not the easiest start to the season but we know it's a very complicated power unit,'' Button said. ''In terms of the problems we had today, we have our head around it and understand the issue.''
He would not say what that is.
''I can't tell you that. As I said they're very complicated these days, but it's sorted now,'' Button said. ''We hope for a much more productive day three and day four.'' Testing ends on Wednesday.
The 35-year-old Briton came late to his scheduled news conference because he had to take an anti-doping test, his first for ''about two years,'' he said.
''We normally have a doping test once a year - a WADA (World Anti-Doping Agency) test - at home or wherever you are, but I (hadn't) had it for a little while,'' Button explained. ''Mine was only a (urine) test, some other people had blood tests, I think.''