Stoffel Vandoorne only managed 34 laps in the morning session
McLaren-Honda had problems with their engine during last week's first four days of testing ahead of the new season although their pace impressed at times.
But the mounting pressure of persistent issues saw the Japanese engine manufacturers part company with consultant Gilles Simon, who worked with Ferrari during their dominance of F1 in the early 2000s.
However, the change appears to have made no immediate difference to their fortunes as driver Stoffel Vandoorne's running was severely limited on the first morning of the second testing block.
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The Belgian driver is desperate to log as many laps as possible ahead of his first full season in F1 but was limited to just 34 before lunch – only Renault's Jolyon Palmer spent less time on track.
McLaren are understood to have identified a issue with the power unit and opted for a full change as opposed to battling the problems further.
It means the team, who will have Fernando Alonso alongside rookie Vandoorne in 2017, have already made at least six engine changes.
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The problems will not please new executive director Zak Brown, who took over from Ron Dennis in November and has been charged with winning the team's first Constructors' Championship in 19 years.
"A championship isn't far away," Brown promised at the launch of the new car last month.
McLaren managed some laps in the early morning before encountering more problems
Fernando Alonso was forced to watch as McLaren struggled on
However, he also conceded that progress was unlikely to be immediate.
"So while we're doing all the right things I think for success, we need to keep moving up the grid," Brown added.
"But I would caution [that] we are not going to be winning any races next year, I don't anticipate.
"So we need to be measured on forward progress, as opposed to ultimate results in 2017."