Originally Posted by Black94Snake
Because forcing the car into OL at idle is really going to help things....
If the o2 sensor is screwed up it will.
If forcing the car to open loop fixes the problem, you know the o2 sensor was sending false readings and causing the ECU to add false corrections, which means either the o2 sensor is dead or you have a pre-o2 exhaust leak. If forcing the car to open loop does not change things, you know the o2 sensor is fine, and replacing it would be a waste of money.
It's called debugging. Swapping out parts until the problem goes away should only be used as a last resort when all other methods have been exhausted. AFR problems are easy to diagnose without arbitrarily replacing anything.