check engine light and tuning

[lncatch]

ok, so ive searched hi and low for the answer to the question...does a check engine light make the car run in open loop? i cant seem to find the answer anywhere. I am trying to find out if the check engine light will affect my tuning. I have a small exhaust leak, doesnt cause any performance issues but im pretty sure its causing my check engine light. i need to have the wideband in the few part to get accurate readings and my lower o2 sensor for the car is behind the leak...yes i know i need to fix it at some point. so really what i want to know is does the cel make the car run in open loop therefore not allowing me to see any modifications to the map since from what i understand it runs in closed loop normally.

 

[yudkib]

some of them do - i believe that it has to do with what the light is for. an exhaust leak would impact your O2 sensor, in which case, the ECU goes to cat efficiency mode. pulls timing like a mother and wont dump fuel like the traditional MSP open loop until around 4200 RPM.

 

[lncatch]

ok, im thinking its not getting enough flow because it runs fine farther up in the system, i think im just gonna throw the electrical mil eliminator on the o2 sensor rather than the non foulers i got on there now