The Elektron system was turned off September 20 after high temperature damaged its rubber seals.
"You can be absolutely calm, the atmosphere at the station is all right," Pavel Vinogradov said.
"The Elektron oxygen regeneration system failed a week ago. Yesterday we replaced one of the system's blocks, checked the electrical scheme, today we will check the thermal control system and hope to turn [Elektron] on tomorrow."
He said the system's failure caused smoke at the ISS, but that the crew easily dealt with the problem. He added that experts at the Mission Control Center were currently examining the causes of the system's failure.
This is not the first time the oxygen regeneration system failed on the ISS. Williams and Vinogradov replaced a hydrogen disposal valve on their first spacewalk to ensure uninterrupted oxygen generation and the disposal of used hydrogen.