When you say front, are you talking about the very front O2's or the ones behind the cats?
The fronts wouldn't be effected by the cats because 1. They're in front of them, and 2. Running hfc's makes you run leaner and not richer. It's not like you where dumping raw fuel on them and fouling them out.
Now if you're talking about the rear O2's, you're still leaner than OEM but you could be flowing more fumes by them in volume even though its leaner fumes resulting in an early foul. But good news is 1. Just buy any O2 from napa and cut and splice the wires into the correct spots (way cheaper!) as long as its a 4 wire O2 and design/thread pitch etc. or 2. Just get a tune and turn them off because you don't need them. The rears are just there to tell you your cats ****** up.
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