The front O2 sensor tells to ECU to make the mixture lean or rich depending on gas content. The rear O2 tells the ECU how well the CAT is working.
The fact that you get the message you get tells me that the rear sensor is working and the front sensor has a problem.
Of course, the problem could very well be that unmetered air is entering the intake manifold, which causes the ECU to enrich the combustion chamber with longer pulses from the injectors. Always check the vacuum lines, elbows, intake manifold gasket, injector seal leaks, etc.
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