Kimi Raikkonen endured a frustrating first practice ahead of the Formula One season-opener in Bahrain when his McLaren was hit by electrical failure.
Raikkonen's McLaren crawled to a halt on the 12th lap of practice
The Finn, whose title chances were ruined by failures last season, broke down on his 12th lap of the session.
Michael Schumacher set the second fastest time behind Honda test driver Anthony Davidson, with Renault's world champion Fernando Alonso fifth.
Jenson Button was ninth for Honda with new team-mate Rubens Barrichello 18th.
Red Bull's David Coulthard was 19th.
Raikkonen's team-mate Juan Pablo Montoya also complained of problems with his car.
"We don't know what is going on but running on the same revs I am about seven kilometres per hour down on Kimi with the same aerodynamic package," the Colombian said.
"Unless we want to finish about 10th or 12th then we should think about changing the engine to be honest but it is not my call.
"I think we will be OK in the race but on one lap we are struggling."
An engine change incurs a 10-place penalty on the grid, although McLaren played down the problems.
"It was a bit frustrating we had an electronic box failure on Kimi's car which is the first in years," said team boss Ron Dennis.
"It's been several years, literally, since we've had that sort of failure so I'm not too concerned."
Schumacher said he believed Renault were favourites for victory in Sunday's race.
"I think that compared to the other teams, we are looking competitive," said the German.
"What is clear, however, is that Renault seem to be very quick straight out of the box which gives them an edge."
The Honda-powered Super Aguri team, who faced a race against time to get their two cars to Bahrain, clearly struggled for pace.
Takuma Sato got the better of Yuji Ide as they battled to avoid last place, but the former BAR driver's best effort was over six seconds slower than Davidson.
Bahrain GP second practice session times:
1. x-Anthony Davidson (GB) Honda 1min 31.353sec
2. Michael Schumacher (Ger) Ferrari 1:31.751
3. x-Alexander Wurz (Aut) Williams 1:31.764
4. Felipe Massa (Brz) Ferrari 1:32.175
5. Fernando Alonso (Spa) Renault 1:32.538
6. Vitantonio Liuzzi (Ita) Scuderia Toro Rossa 1:32.703
7. x-Robert Doornbos (Mon) Red Bull-Ferrari 1:32.926
8. Giancarlo Fisichella (Ita) Renault 1:33.215
9. Jenson Button (GB) Honda 1:33.226
10. x-Robert Kubica (Pol) BMW Sauber 1:33.244
11. Christian Klien (Aut) Red Bull-Ferrari 1:33.557
12. Kimi Raikkonen (Fin) McLaren-Mercedes, 11, 1:33.577
13. Juan Pablo Montoya (Col) McLaren-Mercedes 1:33.726
14. Nick Heidfeld (Ger) BMW Sauber 1:33.848
15. x-Neel Jani (Swi) Toro Rossa-Cosworth 1:33.900
16. Scott Speed (US) Toro Rossa-Cosworth 1:34.284
17. Mark Webber (Aus) Williams 1:34.333
18. Rubens Barrichello (Brz) Honda 1:34.384
19. David Coulthard (GB) Red Bull-Ferrari 1:34.432
20. Tiago Monteiro (Por) Midland-Toyota 1:34.459.
21. Nico Rosberg (Ger) Williams-Cosworth 1:34.953
22. Ralf Schumacher (Ger) Toyota 1:35.170
23. x-Markus Winkelhock (Ger) Midland-Toyota 1:35.686
24. Jarno Trulli (Ita) Toyota 1:35.898
25. Jacques Villeneuve (Can) BMW Sauber 1:36.264
26. Christijan Albers (Ned) Midland-Toyota 1:36.314
27. Takuma Sato (Jpn) Super Aguri-Honda 1:37.588
28. Yuji Ide (Jpn) Super Aguri-Honda 1:39.021
x denotes test driver