Nico Rosberg was once again restricted to less running than he would have liked at Jerez due to more reliability concerns for Mercedes.
Rosberg lost time on the opening day of the test due to problems with the rear wing, the gearbox and a sensor. Michael Schumacher took over on Friday, covering 112 laps and setting the quickest time.
Schumacher ran trouble-free again yesterday, doing 114 laps and finishing the day in third place - with much improved long-run consistency - before handing back to his team-mate for the final day.
Rosberg was only 18 laps into today's programme when he stopped on track again, and the Mercedes team performed an engine change as a precaution.
He was restricted to a total of 45 laps and ended the day seventh. When asked if it was a better day than Thursday, he said: "Not really.
"Today was a bit difficult, especially because of the precautionary engine change to make sure we could complete the running in the afternoon.
"Every lap helps us to learn a lot, but unfortunately not as much running as I'd have liked today."
Rosberg added that he was still optimistic that Mercedes would get on top of its issues.
"It's a steep learning curve and there is a lot of hard work ahead, but with the developments that we have to come, I am confident that we will continue to progress," he said.
Team boss Ross Brawn acknowledged that reliability had to be addressed.
"The four days in Jerez have proved to be extremely useful for the team, although the test has not been without its challenges," he said.
"We had good reliability for the middle two days which allowed us to accumulate a lot of mileage, however there are issues to be resolved so that we can maximise our track time for the remaining two tests.
"Both Michael and Nico continue to feel comfortable in the car and our priorities remain on reliability, developing our understanding of the tyres and the KERS, whilst simultaneously pushing on with the developments still to come."