Red Bull's 2014 troubles continued on the first day of the second pre-season test in Bahrain.
The world champions, whose first test was disastrous, completed just 14 laps with Sebastian Vettel at the wheel.
Red Bull were delayed by five hours because of problems building the car. When they did start running it broke down with over one hour remaining.
Force India's Nico Hulkenberg was fastest from Ferrari's Fernando Alonso and Mercedes' Lewis Hamilton.
McLaren's Kevin Magnussen was fourth quickest from Vettel, who was 3.344secs off the pace.
It is not yet clear what caused the Red Bull to stop on track but there was a burning smell coming from the car when it was returned to the pits on the back of a lorry.
Red Bull have brought two different bodywork packages to Bahrain in an attempt to find a solution to the chronic overheating that, along with severe problems with the Renault power-unit, caused their troubles at the first test.
"Obviously we're not happy with where we are right now but we still have a long way ahead of us," said Vettel.
"It's difficult to judge where we are. I haven't had much of the car yet. The first gut feeling is OK but we need more to judge the car. It is not that easy to find a quick fix. I think we understand the problems but it's not easy to find a solution.
"We had a problem with temperature in Jerez which we seemed to fix, at least with the couple of laps we could do. But very often you fix one problem and another one pops up."
They and the other Renault teams continue to struggle with power unit-related problems.
Renault arrived in Bahrain admitting it had not completely solved the problems that had made the first test a waste of time for all their teams.
The Red Bull, Caterham and Lotus cars were able to do more laps than Renault managed at the first test, but they are still being afflicted by some problems
Lotus completed only seven laps on their car's first public test day and Toro Rosso just five. Of the Renault-powered teams, only Caterham managed decent mileage, with reserve driver Robin Frijns doing 58 laps.
"There is stuff to do on the Red Bull Racing side in terms of temperature and reliability around the car and there is stuff to do on the Renault side, but it's not fair to separate the two," added Vettel.
"We are a team. We have been very successful in the last years together. This is not the start we were hoping for but we still have some time and clever people on board who hopefully can fix the problems."
Renault engineering chief Remi Taffin said he believed Renault had made "quite a step", adding: We could run the power-unit as we wanted. It's still not the level we want but at least we gave a baseline."
And he seemed to imply Red Bull's problems were car-related rather than anything to do with the engine, saying: "Basically we were waiting for them. We went out when they decided to go out and we finished when they decided to finish."
Hulkenberg's fastest time was a lap of one minute 36.880 seconds, 0.999secs faster than Alonso and 1.028secs up on Hamilton.
The German's lap - set on the fastest 'soft' tyres, which no other driver used - was 2.4secs off the quickest lap in the first practice session at last year's Bahrain Grand Prix and just under 0.1secs faster than the fastest lap set by Vettel on his way to winning that race.
Despite completing 74 laps, Hamilton was not completely happy with how his testing went as he was troubled by a braking problem
"We didn't get as much running as we would like but it's only the first day," he said.
"It's very difficult to know where we are in terms of performance but in terms of going the distance we are there at the front. We are doing lots of laps but it is still a massive challenge for us. We are still finding issues but it looks like others are too."
The pace set by the leading contenders should quieten concerns in some quarters that the cars will be too slow this year following the major regulation changes that have been introduced this year.
F1 has adopted 1.6-litre V6 hybrid turbo engines and a fuel restriction, as well as significant changes to the aerodynamic rules.
Engineers predict the cars will be in the region of two seconds slower than last year by the time of the first race in Melbourne on 16 March, and will continue to evolve rapidly through the season.
At least half a second of that deficit is caused by tyre supplier Pirelli using harder rubber compounds to cope with the greater demands of the new engines.