Red Bull's pre-season problems showed little sign of easing in Bahrain testing with four-time world champion Sebastian Vettel managing only 14 laps on the first day.
The Renault-powered team, who did the same number of laps over four troublesome days in last month's first test in Jerez, said a mechanical issue sidelined the German at the Sakhir circuit.
"Obviously we're not happy with where we are now and we have a long way ahead of us," said Vettel in a team statement. "It's not easy to find a quick fix but I think we understand the problems.
"However, very often you fix one problem and another pops up - which is what happened."
Formula One has a new V6 turbocharged power unit with energy recovery systems this year, replacing the old V8s, and Renault have had more problems than Mercedes and Ferrari in getting their teams up to speed.
Red Bull's race engineering co-ordinator Andy Damerum said there had been a big improvement since Spain but the team was playing catch-up with rivals.
Germany's Nico Hulkenberg was fastest on the first day for Force India after putting in 78 laps. He was followed on the timesheets by Ferrari's Fernando Alonso, who did 64, and Mercedes' Lewis Hamilton with a healthy 74 under his belt.