McLaren says it found some consolation in knowing that the issue preventing it from running on the final day of testing at Barcelona was "relatively minor".
Fernando Alonso managed just three untimed laps in the morning before he had to stop for good due to a coolant leak in his car.
McLaren's engineering boss Matt Morris said the problem was small, but with the leak being in a place with difficult access, it took a long time to reach it.
"Unfortunately, the coolant leak we experienced on the car today was in a tricky position, which meant it took longer than usual to locate and fix," said Morris.
"It's always frustrating when a small issue hampers your running for much of a day, but thankfully it's relatively minor and we can now look to preparing the car for the second test on Tuesday.
"Testing is testing, and it gives us the opportunity to identify and work on reliability issues before we start the season."
Morris said that, despite today's setback, McLaren had a solid base to work on in next week's test.
"Over the course of the first four days in Barcelona we've done a good amount of running and already learned a lot about our package, which should stand us in good stead for next week.
"We're definitely ahead of where we were last year in terms of system checks and integration, so at the second test we'll be able to focus more on race simulations and set-up, as well as characterising our package's performance."