In our old car, the catalytic converter wasn't effective, and the cost of fixing it was more than the car than the car was worth. The check engine light was on constantly as a result, but it was easy to ignore. Judging by the smell of that car's exhaust, it was definitely in the 25%.
Bad air-fuel ratios also cause lots of pollution due to unburned fuel being sent out the tailpipe. I would think that this is less common with fuel-injected vehicles, but there are still plenty of old carburated vehicles on the road, especially where road salt isn't used.
Bad air-fuel ratios also cause lots of pollution due to unburned fuel being sent out the tailpipe. I would think that this is less common with fuel-injected vehicles, but there are still plenty of old carburated vehicles on the road, especially where road salt isn't used.