Red Bull's truncated third day of running in the Bahrain Formula 1 test was caused by a car problem, not an engine issue, according to Daniel Ricciardo.
The Australian managed only 28 laps on Friday, ending up ninth fastest, six-and-a-half seconds off the pace. He blamed the lack of running on mechanical problems.
Asked if it was an engine malfunction that had brought Red Bull's day to an early end, Ricciardo replied: "No, this afternoon it was purely mechanical.
"We could have liked to have done more laps, but it's getting there.
Bahrain test day three report
"Seb [Vettel] did nearly 60 laps [on Thursday] and we're building it up slowly.
"Yes, we had a mechanical issue this afternoon so we decided to call it off for the day and get it fixed for tomorrow, so hopefully we get a full day of running."
Red Bull has completed 101 laps over three days in Bahrain, on top of the 31 it managed during the first pre-season test at Jerez two weeks ago.
RENAULT 'MAKING PROGRESS'
Renault head of track operations Remi Taffin confirmed that the problems suffered by both Red Bull and Lotus were not engine-related.
When asked by AUTOSPORT if the issues that halted both teams' running were due to the car or the engine, Taffin said: "I know what the problems were, but I can't say. [It was] on the team side.
"We are making progress, we can afford to do many laps a day.
"It's not true to say we have trouble-free days, this is testing and we are still getting problems, but not as major as before."
Ricciardo is confident that Red Bull has made progress with its car and that the team will be in good shape for the season-opening Australian Grand Prix next month.
"Definitely we have made progress from Jerez and we knew it wasn't going to happen overnight," he said.
"I think we're making the steps we needed to make.
"Obviously, it's still not enough but we're getting there so hopefully tomorrow and next week and coming into Melbourne we should be on a good level."