Had a code for the DTS 0420
3way catalyst low efficient, does this mean i should just replace the cats, and how many cats does the car have

 

P0420 Normally means new cat.conv. there is only 1.Oxygen sensor(s) sometimes if they won't come out.Ecm and cat.conv covered under fed.emission warranty from 8yrs or 80k miles from original in service date so if its free if this applies to your car.To find out in service date if you did not get car new, dealer will need last 8 digits of seriel number from that you should be able to get in service date,open recalls,and if warranty is in effect.Due to emission laws you may not be able to install aftermarket cat and have to oem

 

I have a 2003 Deville with 152k. I got that code and replaced all 3 oxgen sensors and Cat. I still get the code and a smell. I can't see any damage in the exhaust pipes.

 

Before believing DTC P0420 and replacing the cats:
Diagnose all other DTCs.
Inspect the exhaust system for leaks or damage.
Inspect the oxygen sensors
Inspect the wiring and connections for damage or bad connections.
Clear DTCs and operate the vehicle a couple of times to reset the DTCs.
If P0420 is still there after the others are fixed replace the catalytic converter.