I took out my oxygen sensor and it's a bit black... no error codes.... only 60k miles (100K km)

Should I replace it or is this okay if it's like this?.

 

Don't know why you would replace a sensor that doesn't have a issue, it wouldn't hurt to replace it as long as you replace it with a OEM sensor.

 

I asked because I just remember reading somewhere once said that when they get black like mine they don't monitor as well and your gas mileage suffers even if you're not getting any error codes. You're saying this isn't true and that they either work or they don't work?

 

I wouldn't worry about it unless its throwing a code. If your worried about mileage clean your throttle body and replace your sparkplugs to start.