Fernando Alonso suffered fresh frustrations at Barcelona on Thursday morning when unspecified technical issues on his McLaren-Honda left him confined to the garage.
McLaren had got off to an encouraging start after completing 254 laps on the opening three days, before Jenson Button was stopped with a fire caused by an hydraulic leak on Wednesday afternoon.
With work taking place overnight, McLaren had been able to get Alonso out of the garage almost immediately at soon at testing began on Thursday morning.
But after three installation laps, the team discovered some issues with water pressure so set about more detailed checks on the car to see what was wrong.
Mechanics were seen removing the engine cover and working at the rear of the car, meaning Alonso was forced to sit out and wait.
Honda Tweeted that it hoped the Spaniard would be back in action before the end of the day.
Honda's F1 chief Yusuke Hasegawa admitted earlier this week that the Japanese manufacturer had not yet fully signed off the reliability of its parts for the Australian Grand Prix.
"In general we checked our reliability on the dyno, so we have already confirmed our engine cell is running on the dyno now for 4000km," he explained. "But of course we have to check in the car as well.
"Reliability is one of the very important things to update from last year to this year. But as I mentioned, still we have some minor problems for the parts, so we have to confirm every part is alright for the race."