But why replace 4 when only one is failing?
You can make an argument for the two matching fronts (they usually are the first to fail) just because they get the same exposure and the car is on the lift and the muffler system is cool and you may save money (PS, any muffler shop can do a single O2 sensor for 20 minutes of labor, no special Porsche tools/knowledge needed. The local muffler shop did mine.).
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