Vettel breaks down during testing

Red Bull team boss Christian Horner says he does not know how long it will take to solve the reliability problems that have blighted their pre-season.

The constructors champions have had car and engine issues and done far fewer test miles than Mercedes and Ferrari.

Horner said: "I'm very confident we'll overcome the issues; the question is when and how quickly."

He admitted Red Bull were "on the back foot" ahead of the first race of the season in Australia on 16 March.

Red Bull on back foot - Horner

"We're trying to go to Melbourne in as good a shape as we can, but we've probably covered only about a third of the mileage of some of the other teams," added Horner.

"But we're pushing and we want to catch up as quickly as we can."

Red Bull suffered another blow on the penultimate day of pre-season testing on Saturday.

They had hoped world champion Sebastian Vettel would get a good day's running after team-mate Daniel Ricciardo had achieved the team's longest run of the winter, at 20 laps, on Friday afternoon.

But after spending two-and-a-half hours in the garage at the start of the day, the car broke down on Vettel's first lap out of the pits.

When the German tried to go out again 45 minutes later, the car stopped in the pit lane and had to be pushed back to the garage for more work. The team failed to get out again for the rest of the day.

Asked what Red Bull's realistic goal for the first race was, Horner said: "Seeing the chequered flag. That would be a start. But first of all you have to finish and performance-wise, we have a good car.

Formula 1 testing: Red Bull struggle in Bahrain

"I have every confidence we have a good chassis. We just have to make sure we get the most out of what we have going into Melbourne and develop that in subsequent races."

Horner said some of Red Bull's failures were down to the team and some down to the Renault power-unit, which has been unreliable and problematic for all four of the French manufacturers' teams.

Asked if he had the same confident in the unit as he had in the car, Horner said: "It is still very early days.

"We have every confidence in Renault; all our success has come with them. There are some very clever people in [their French base in] Viry and I'm sure they're going to resolve the issues."

He said some of the difficulties were caused by Red Bull's focus on winning their fourth consecutive title in 2013.

"Our start on this year's car was perhaps later than others because we were pushing for last year's championship," Horner said.

"When you're packaging the car and trying to optimise it, sometimes you go a bit over the limit and have to come back a little bit. That's what testing is all about."