O2 Sensor/ECU Question

If the O2 sensor detects a problem, does the ECU put the fuel/air mix into a “safe” mode when it runs rich? Less gas mileage but ” no” damage to engine?

 

Is this not something you could've just asked in your original O2 question thread?

 

I guess so, it seemed like a more general question.

 

In my experience, no. The ECU does not default to 'safe mode' when an o2 sensor throws a code. When o2 sensors start to fail, they usually start reporting a lean condition (low voltage), so the ECU increases the amount of fuel (as can be seen in the fuel trims) which hurts your fuel economy. If you unplug the o2 sensors (<- not recommended!) the ECU will receive no voltage reading from the o2 sensors and you will see an increase in the fuel trims.