I have a 04 wrx, it has no cat converter and a 3" exhaust, engine light came on, a Mac code reader indicated fault code P0420, "cat efficiency below threshold, bank 1", it has been reset and keeps coming on, I've been told changing the sensor won't help. What next? Is there a "blocker?" or something else that can replace the sensor, so the check engine light stays off and the car performs like it should??