Honda Formula 1 boss Yusuke Hasegawa has said he is "worried" about the company's lack of reliability in the first pre-season test.
Hasegawa said McLaren's engine supplier still did not know what had caused an engine failure during the test.
He added he could not be confident that the problems would not compromise the start of McLaren-Honda's season.
"The mechanical issue, I don't know enough yet to be confident about that. Of course I worry about that," he said.
Hasegawa, who was talking to Spanish television station Movistar, said that because the cause of the failure was not yet known, Honda could not be sure what specification of engine would be used in the final pre-season test.
The plan was to use a new specification aimed at the first race of the season in Australia on 24-26 March.
The engine failure, on the second day of the test, was one of a series of problems that led Honda to use at least five engines over the course of four days in Spain.
There was an oil tank problem which led to an engine change and restricted Fernando Alonso to just 29 laps on the first day of the test.
A third new engine was fitted overnight before the Spaniard's team-mate, Stoffel Vandoorne, started testing on Tuesday. But it failed after 29 laps and needed to be changed for a fourth engine that day. The Belgian was able to run again later and complete a total of 40 laps.
The Honda ran more reliably on Wednesday and Thursday but the company used at least one further engine during those two days. A spokesman would not confirm the exact number of engines used during the test.
Asked whether the problems had affected Honda's relationship with McLaren, Hasegawa said: "In the tests, of course, this is a stage we need to overcome a lot of trouble. So sometimes we need to argue or we need to have constructive discussion, but I think we are doing a very good job and we have a very good relationship."
Despite the problems, Hasegawa said Honda's ambitions to improve on last year's sixth place in the championship remained.
"We need to gain further steps this year to close on the front-runners," he said.
Honda has fundamentally redesigned its turbo hybrid engine for this season, and is now using a similar architecture to that used by Mercedes so successfully since 2014.
"We have modified the package so the engine is lighter and the centre of gravity lower," he said. "This gives us a big benefit of the behaviour of the car and we change the internal combustion engine to extract more power."