What do your car do when your o 2 sensor is bad?

What do your car do when your o 2 sensor is bad?
Technically, when there is a fault such as an O2 sensor failure, the engine management system (the computer) goes into "Open Loop" condition. This mode is normal while the engine is warming up but causes poor fuel economy and reduced performance. Once the engine has warmed up to normal operating temperature it will switch to a Closed Loop condition. You always want the system to go into closed Loop under normal conditions.
O2 sensor failures will sometimes cause the "Check Engine" light to come on, sometimes not. It is always best to have the system checked for fault codes if you suspect a problem like this.