What happens when the oxygen sensors dont work?

They send a false return voltage signal to the ecm (computer).

The ecm then sends too much or too little fuel to the cylinders,

causing poor performance, wasted fuel, overheating, etc.

They don't send a code until they're way off, so it's better to

replace at 75 thousand miles, or so, even if no code's present.

It will pay for itself via better fuel economy and performance.