The front ones are harder to get to, but in my opinion are more critical than the rear. The fronts are used to actually set the mixture, but the rear are really just there to analyze the operation of the catalytic converters.
They are both heated sensors and have specific length wiring. I don't know of any generics that will work.
Can you post what code you had and the fault code description.
The front ones aren't all that bad to do if the exhaust system is completely cool.

Gilly

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