The British team were keen to bounce back from last week's disappointing opening test in which a number of engine issues plagued their running.
But on the opening morning of the final week of testing, an electrical fault restricted Stoffel Vandoorne to only 34 laps of Barcelona's Circuit de Catalunya.
And when asked how much of a strain the McLaren-Honda partnership was now under, Eric Boullier, racing director for the British team, replied: “Maximum. We are in Formula One, we are racing, and we have to perform. So the pressure is obviously huge and we put the maximum pressure on all of our relationships with Honda, and the same from them.
“We cannot put a step wrong. We need to be able to deliver the best car as well, so this is both sides.”
The latest setback on Tuesday will come as a significant blow, both to McLaren and their beleaguered engine supplier, with just three full days of testing remaining before the season gets under way in Melbourne on March 26.
McLaren, who have won a combined total of 20 drivers' and constructors' championships, are looking to overturn a miserable run of form which has seen them fall down the grid in recent seasons.
They have not won a race since Jenson Button triumphed at the 2012 Brazilian Grand Prix, and their renewed partnership with Japanese manufacturer Honda, which is now entering its third campaign, has been thoroughly underwhelming.
Their new car, the first since long-serving chairman Ron Dennis was forced out of the team, has been hampered by reliability issues and only Toro Rosso completed less mileage than McLaren at the opening four-day test in Barcelona last week.
While Vandoorne ultimately managed 80 laps on Tuesday - the most McLaren have managed in a single day of testing this year - he was nearly three seconds adrift of Felipe Massa, who surprisingly topped the order for Williams.
Double world champion Fernando Alonso, who is entering the final year of his contract with the British team, has been non-committal on his future beyond this season. He returns to the cockpit on Wednesday.
Aside from McLaren's woes, Massa completed a mammoth 168 laps for Williams and set the fastest time. His best lap of one minute and 19.726 seconds saw the Brazilian finish ahead of Red Bull's Daniel Ricciardo and the Ferrari of Sebastian Vettel. Lewis Hamilton, accompanied by his pet dog Roscoe in the paddock on Tuesday, was fourth fastest.
The final test concludes on Friday.Reuse content