The stock 02 sensor is a narrowband sensor that essentially functions by telling the ECU that the A/F ratio is either ok, lean or rich. As long as the o2 sensor is in the ok range the ECU is happy and it takes no action.

If the A/F ratio becomes lean it informs the ECU and the ECU then issues a command to the fuel management system to squirt more fuel into the cylinders.

Likewise if a rich condition is measured the ECU tells the fuel system to squirt less fuel into the cylinders.

Many posts have cautioned about potentially damaging an engine by running lean after making significant mods to the intake/exhaust.(non FI)

Question: If a lean or rich condition occurs doesn't the ECU issue a warning (engine light) before any serious damage occurs?

 

I believe it would if it couldnt get it under control .Meaning if it couldnt make the A/F leaner or richer...what ever the condition it was in