Toro Rosso Formula 1 drivers Pierre Gasly and Brendon Hartley are set to start Sunday’s United States Grand Prix from the back of the grid following a decision by Honda to change the power unit on both cars for the Austin weekend.

Honda introduced its updated specification power unit for the Russian Grand Prix last month before putting it through a maiden race run at Suzuka two weeks ago.

Despite positive feedback from both Gasly and Hartley, Honda confirmed ahead of opening practice for the United States Grand Prix on Friday at the Circuit of The Americas that the Toro Rossos would both take a new power unit at some point during this weekend.

“On both cars, the new units will feature minor modifications to the specification when compared to that used in the Japanese Grand Prix,” Honda F1 technical director Toyoharu Tanabe said.

“After examining the power units used in Suzuka, we had some concerns and so we have decided to make these changes, which we believe is the best strategy for Scuderia Toro Rosso Honda for the remaining races of the season.”

Gasly’s car was fitted with the new power unit ahead of opening practice at COTA, with the FIA confirming in its regular Friday technical bulletin that the Frenchman had taken a number of new parts for the session.

Hartley will continue to run with an older power unit on Friday before taking a new power unit prior to Saturday’s running.

With both drivers already well beyond their season allocation for power unit elements, they will receive penalties that drop them to the final row of the grid for Sunday’s race.