Just had DTC p0420 come up on my 03 concorde lxi 3.5l was told there are various things it could be what's the most likely culprit or most common?

 

This code typically indicates the Catalytic Converter (Bank 1 or 2, or both) is inefficient and there is not the correct level of oxygen after exhaust leaves the converter.
DO NOT JUMP IN AND REPLACE THE CONVERTER$$$$$
Are there any other Symptoms like poor acceleration (could indicate clogged converter) or just the code?
It all has to do with the air /fuel mixture AND the converter condition.
Start with the basics First. : Air Cleaner, PCV and Spark Plug maintained and in serviceable condition.
Could be just a glitch, erase code and see if and or when it returns.
(Maybe run a can of Seafoam or other alleged converter cleaners thru the gas tank but usually not effective)
Some folks may recommend replacing O2 Sensors. If no other codes don't bother. (If you do DO NOT USE BOSCH, NGK or NTK only in an Chrysler product)
Final Note : Spacers/Extenders work and will "fool" the ECM and eliminate the code.
Remove post converter sensors, reinstall with spacers and enjoy the code free experience
. https://300mclub.org/forums/viewtopic.php?t=35242

https://www.google.com/search?q=oxyg...hrome&ie=UTF-8

 

No other codes. Only other thing I noticed is the harmonic balancer is wobbling. The code popped up during a rain storm

 

Slight Wobble somewhat common on the 3.5. Code Popped up during rain storm, bet the Catalytic Converter got splashed/cooled causing PCM throwing code.
Erase Code, bet it doesn't come back.

 

I'll give it a try thanks

 

I tried to post a video of the wobble. But it won't let me. It says incorrect format apparently can't use mp4.

 

If that is you on Facebook with 309,000 miles, you defiantly need to replace that damper.
How many miles/age on the timing belt/water pump? 7 yr/100k change interval.

 

Yes it is, timing belt and water pump were replaced at 300k. The people I bought it from bought it in 2003 with 18500 on it and kept up on scheduled maintenance. It's had the timing/waterpump changed every 100k

 

As FIREM noted, do not replace the cat. Replacing the cat is the last step in diagnosing a p0420

P0420-CATALYTIC 1/1 EFFICIENCY
When Monitored: Engine Run time greater than 90 seconds. Engine Coolant greater than
70°C (158°F) Vehicle speed greater than 20 MPH and less than 55 MPH. Engine Speed
greater than 1216 RPM and less than 1952 RPM.

Set Condition: As catalyst efficiency deteriorates, the switch rate of the downstream O2
sensor approaches that of the upstream O2 sensor. If at any point during the test the
switch ratio reaches a predetermined value a counter is incremented by one. One Trip
Fault.

POSSIBLE CAUSES:

GOOD TRIP EQUAL TO ZERO
VISUALLY INSPECT CATALYTIC CONVERTER
EXHAUST LEAK
ENGINE MECHANICAL CONDITION
AGING O2 SENSOR
CATALYTIC CONVERTER

I have had a lazy/failing(slow to react) upstream O2 sensor cause a P0420 dtc. The actual O2 code was for a failed heater circuit). The pcm compares the output of the upstream O2 sensor with the output of the downstream O2 sensor. If there is not enough difference due to age/laziness, the p0420 is set. If replacing the O2 sensor, use OEM/NTK as the first choice. Denso has been know to work but Bosch are a hit/miss proposition and should be avoided.

 

Got the pulley replaced the rubber between the inner and outer parts was missing in spots. Lucky it didn't come apart while driving.