Honda has decided to change the engine in Stoffel Vandoorne's car on the first day of the final test in Spain after finding an electrical problem.
It is the latest in a series of problems with Honda's newly redesigned engine after Honda used at least five in last week's first pre-season test.
Vandoorne ended the morning eighth fastest, 2.972 seconds off the pace.
A Honda spokeswoman said: "In the interests of maximising lap time, we sped to replace the power-unit and investigate the issue further once it is out of the car.
"The PU we are using is the same specification as the one run at the test last week."
Follow live text commentary on the first day of the final pre-season test
Honda has said it has an upgraded engine that it is planning to use for the first race of the season in Australia on 24-26 March. That latest specification is expected to run at some point this week at the Circuit de Catalunya-Barcelona.
The quickest lap of the morning was set by Williams driver Felipe Massa on the super-soft tyres, 0.174secs quicker than Daniel Ricciardo's Red Bull, using the ultra-soft tyre.
The soft tyre is calculated by Pirelli to be 0.8secs a lap slower than the super-soft.
Mercedes driver Lewis Hamilton was third fastest, 0.556secs behind Ricciardo and also on the soft tyre.
Ferrari's Sebastian Vettel was fourth quickest, 0.671secs behind Hamilton but using the medium tyre which is said to be 1.3-1.4secs slower than the soft.
It is impossible to directly compare the lap times even taking into account the tyre off-sets because the teams do not reveal the fuel loads in the cars or the specification in which they are running, both of which can make significant differences to lap time.
McLaren and engine partner Honda have suffered another blow in a troubled pre-season programme.