PO420 code after installing an aftermarket cat

I installed an OBD-II compliant aftermarket cat and new sensors, and I keep getting a PO420 MIL. The dealer told me that they couldn't find anything wrong with the car, but would enjoy selling me a "genuine" GM cat for ~$1500 installed.

Can the ECU settings be changed to work with the replacement cat?

Car: 2001 base Impala 3.4.

No they cannot, second was the420 code there before the cat replacement?

Code caused by aftermarket catalytic converter.
Should have purchased a factory piece

Get an O2 simulator. They are about $40, but it will solve your problem.
http://www.zzperformance.com/grand_prix/products1.php?id=179&catid=107
It shows the engine computer the same reading a factory cat would. These cars are very sensitive to which cat you put on.

put the old o2 sensors back in. reset pcm. see if new sensors are good. o2 sensors will easily get damaged by dropping or wrong wiring etc...

Both sensors are stock and brand new, and I've made sure the wires weren't anywhere near an RF source from the ignition. I certainly hope the dealer would've also informed me if they (or anything else) were bad.

Unfortunately, I threw out the old set but still remember the code being there before I changed them.

I'd go replace this item now, but I think $1500 is unfair for something that I could otherwise find online for about $200 and have my favorite neighborhood indie mechanic install.

Both sensors are stock and brand new, and I've made sure the wires weren't anywhere near an RF source from the ignition. I certainly hope the dealer would've also informed me if they (or anything else) were bad.

Unfortunately, I threw out the old set but still remember the code being there before I changed them.

I'd go replace this item now, but I think $1500 is unfair for something that I could otherwise find online for about $200 and have my favorite neighborhood indie mechanic install.
take numbers off sensors and check that they are the correct oem o2. I suspect that the sensor after the cat is defective or the wiring connector etc remove connector inspect pins etc.. if this checks out ECT SENSOR or COOLANT getting into the combustion chamber can cause this like manifold or head gasket problems

All this sensor business is guess work, a full scan should be done to check the post cat sensor for rate of switch after warmed up and no existing other codes, the post cat sensor should be maintaining a 450-650 mv reading, better done with a graphing scanner to compare switch rates with the pre cat sensor, change parts and experimenting is not the way to fix an OBD11 car.

All this sensor business is guess work, a full scan should be done to check the post cat sensor for rate of switch after warmed up and no existing other codes, thpost cat sensor should be maintaining a 450-650 mv reading, better done with a graphing scanner to compare switch rates with the pre cat sensor, change parts and experimenting is not the way to fix an OBD11 car.
Agreed.
It is well known that these cars do not like aftermarket cats, hence need for the O2 simulator I refered to earlier with an aftermarket cat. Also, the rear O2 sensor is only used by the pcm to check function of the cat. It does not affect mileage, engine life, or driveability.

Correct.

ok i could keep my self from telling you what i think about this problem but i think (im not sure) the code could be caused if the cat is not burning all or enough air/gaz mixture that did not burn during combustion phase cause the emission control system is quite exigent and will pop ur MIL on if there is too much mixture left after it went threw the Cat so if you cat is not giving the oem performance that the pcm have in memory u get a Mil light anyway i would just get an OEM Cat im pretty sure that is your problem however i know Oem cat cost alot maybe u want to look for a used one ... or leave the aftermarket cat and use O2 simulator its very cheap and well u just unplug secondary o2 and plug the simulator and try to put it as far from water and exhaust ... hide it somewere and make sur its gonna stay there ;) the simulator will provide the pcm the same mA signal as a well functioning cat

All this sensor business is guess work, a full scan should be done to check the post cat sensor for rate of switch after warmed up and no existing other codes, the post cat sensor should be maintaining a 450-650 mv reading, better done with a graphing scanner to compare switch rates with the pre cat sensor, change parts and experimenting is not the way to fix an OBD11 car.

I agree, but the original post stated that the DEALER found no problems, this is probably where he messed up . should have had vehicle checked out at a reputable repair shop before any parts replaced. 1500 dollars and the problem is not resolved....o2 sensors are very fraglie and I still believe that the scan will reveal this is defective, that is why I suggested that he put the old ones in ,to see if it changed, how much would that cost , but they were thrown out.

A full scan is still necessary to see what is happening, the area between lite and no lite on is very narrow, you could bounce back and forth with this forever, these type issues require in depth diagnostics, yes putting the sensors back short of a proper analytical diagnosis is one step, but bottom line, looking at what is happening is the right way here.