Check your front exhaust pipe and the exhaust manifold. There might be some crack where air pass through.

When the code P0421 came to my car, I notice some exhaust air coming from the front exhaust pipe (pipe before the cat). I bought the exhaust pipe, installed it, and the code was gone. I just bought the O2 sensor due to the rebate and it somewhat improve the fuel consumption as well as the idle of the motor.

 

Replaced the front cat (400$) and it took the code away.
Done with smog *yay*

 

So you had to replace the whole exhaust manifold? that is where the front cat is (right?)

 

I was able to fix the problem with the code P0421
the problem is q O2 sensor does not heat fast enough in my case was q exhaust manifold was not the original and the sensor is very low. place the sensor closer to the engine exhaust and ready