I have '06 WRX stock build........This is the same complaint I have and I have
http://www.iwsti.com/forums/2290713-post1.html post from another forum from someone else. His solution was replacing the ECU.
All I did was raise up the intake manifold to fix the fuel lines and I went from Lean AFR's to stuck AFR.
New front Denso oxygen sensor installed tonight, No luck. Verified both engine grounds are bolted currently to the Intake manifold.
I can tell from my multimeter that the ECU is not grounding the heater circuit as there is no voltage drop across the pins, just batt voltage on both.
Tomorrow I will be doing a pinout check at the ECU once I get a diagram printed out at work.