97 e420 o2 sensor code

[amosfella]

My grandma's 97 E420 canadian version had the check engine light come on about a week ago. So, I checked the obd2 codes, and it came back with code P0150. So, I cleared it, and told here to tell me if it came back. The next time she was driving it, same thing again. Check engine electronics notification came on again. Same code.
Now, is this most likely caused by a dead o2 sensor, or another problem?? I dont' know when the last time the O2 sensors were replaced, and should the O2 sensors like that be replaced in pairs??