The Doctor had been scrolling the log and had his chin resting on his hand as he did.

"You refurbished the life support systems," he noted without looking up. "That requires turning them off completely first."

It was the reason the Doctor had never gone through and worked on them. Either he'd have to be completely in a space suit that didn't run on the TARDIS's energy, or he'd have to be prepared to lose at least one regeneration in the process.