GMC Repair: catalytic converter, coolant temperature sensor, engine coolant temperature sensor

Question
I have an 05 pontiac grand prix, it has a p0420 code on it.  I changed spark plugs and wires and eventually changed cat with an aftermarket federal emissions cat.  My car is California emissions, is this the reason I still have a p0420 code? I have had a smoke test and compression test, all were fine.  No leaks.  Seen four mechanics, dealer, exhaust specialist, and NTB. No one can help me.

Answer
This could be a few things. The 0420 code means Catalyst System Efficiency Below Threshold (Bank 1). The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

 A code P0420 may mean that one or more of the following has happened:

Leaded fuel was used where unleaded was called for
An oxygen sensor is not reading (functioning) properly
The engine coolant temperature sensor is not working properly
Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
Retarded spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings

Possible Solutions

Some suggested steps for troubleshooting a P0420 error code include:

   Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
   Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
   Inspect the downstream heated oxygen sensor (HO2), replace if necessary

If you did not replace either O2 sensor I would do that. Also, after doing so, disconnect the negative battery cable for at least 30 minutes. If the code comes back, let me know and I will look for other solutions.