Slight emissions question.

For awhile now my check engine light has been on and occasionally my air bag light as well. As for the engine light, I had it checked and it turned out to be my purge control valve. But that's not the real question exactly. I read elsewhere that a check engine light will retard the engine's timing/have it run richer than normal. I know of a knock sensor doing so and it makes slight sense that a check engine light would too. This is interesting because I could swear that I get pitiful gas mileage for a 6 cylinder. Can anyone validate this?