View Full Version : Code P 0420 Catalytic Converter

10-26-10, 09:22 AM
I have replaced the Catalytic Converter on my 2006 DTS
Using a code reader turned off check engine light.
Light comes back on after driving car two or three times.
Checked code and it reads
PO420 Catalysts Efficiency Below Threshold.

Things I have been told:
On a 2006 DTS the check engine should should go off with using a code reader to reset??

Also told could have an O2 sensor bad?? (How would I know?)

The light has been reset 5 fives now.

Any suggestion or thoughts.


10-27-10, 01:11 PM
May want to check the voltages on the O2 sensor.

I had my Catalytic Converter replaced a while back under the CPO warranty as my CEL went on (P0420) and it caused the car to not run right (loping idle and reduced power under acceleration).

10-28-10, 08:42 AM
2006 had a software update to prevent false instances of a P0420.
Faulty oxygen sensors would not cause a P0420 because before the test can run for P0420 all the Oxygen sensor self tests MUST pass.

Did you replace it yourself or have someone do it for you?

Be sure you have no exhaust leaks, if fresh air is getting in there it can affect the diagnostics and set a false cat code.

10-28-10, 10:43 AM
Thanks for the help. I had it replaced at a muffler shop. Not Cadillac service center. I will take it back again have them look at it.

10-28-10, 08:31 PM
They are required by law to provide you with a functioning cat for 5 years or 50,000 miles after they install a replacement.
If the light comes back on, take it back.

04-20-13, 09:56 PM
I believe there is a re flash for the ecm to correct over sensitive calibration