A good question would be: what counts as a defective product? See the red graph on this page for the breakdown of CO2 emissions per module of the Fairphone 2.
Even if all back covers are replaced and most of the bottom modules, it still has little impact because those modules together account for 2% of the CO2 emissions. Add to that a couple screen and battery replacements (the screen was changed in summer 2017, so probably about half of the FP2’s was sold with an old screen.) I don’t recall the number of screen defects being particularly large except for some bright spots, which many people decided to live with.
So you can say, that the average phone was defective for… 5% of its CO2 emissions?
That number in itself still doesn’t mean anything. You don’t know the CO2 emissions saved by people not buying a new phone but repairing their existing one.
The page that I linked at the top of this post has some more statistics (from 2016) that you may find interesting.