ferret

My experience IF IT IS the O2 SENSOR, when the engine is COLD, All is fine. When it warms up, it goes into 'closed loop mode' and sets the mixture ONLY by the front o2 sensor. This is when I have seen failures caused by the front sensor. Unplugging it sets a code, but the hesitation is immediately gone. That's how I test the front sensor.

If you are having an IDLE problem, but runs fine at highway speeds, This I have found to be a sticking EGR at times ... for those Subarus that DO have an EGR.

Mikey aka ferret