Honda has switched Toro Rosso Formula 1 drivers Pierre Gasly and Brendon Hartley back to its previous specification power unit for the remainder of the Russian Grand Prix weekend.

Honda brought its updated 'C-spec' F1 power unit to Sochi to debut at the Russian Grand Prix, with both Gasly and Hartley confirmed to have grid penalties as a result of taking the upgraded parts.

The Toro Rosso drivers ran the updated power unit on Friday in FP1 and FP2, but Honda confirmed ahead of final practice in Sochi that both would be reverting to the previous-spec power unit for the remainder of the race weekend.

"Having studied all available PU data after FP2, we realised that, although the performance was encouraging, there is still some work to do in some areas such as calibration and matching the PU to the chassis," said Honda F1 technical director Toyoharu Tanabe.

"We have decided to revert to the older specification PU on both cars for the rest of the weekend.

"We believe we can complete the work before the Japanese GP, so that we can run the new specification PUs on both cars from then on."

The FIA stewards confirmed ahead of FP3 that Hartley has taken another new turbocharger, MGU-H and MGU-K for the rest of the weekend, adding to his season total.