My 08 Alpha with 76K miles has a check engine light with a P0420 code which is: "Catalyst System Efficiency Below Threshold (Bank 1)". After doing some general research it looks like it is most likely that the catalytic converter needs to be replaced, however it could be an O2 sensor or an exhaust leak among other things.

The car seems to be running fine overall. I'm curious if anyone else here has had this code and what the actual problem ended up being.

 

I have had the same problem .. its about the O2 and Carbon level , check the sensors and check if there is a bad smell going out of your exhaust system .. so it could be leakage or expired sensor .

 

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). Catalytic converters are not normally a "wear" type item, that is they are not deisgned to wear out and need replacement. If they have failed, it is likely due to something else that caused it to fail.

Causes
-Leaded fuel was used where unleaded was called for
-A damaged or failed oxygen sensor (HO2S) Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
-The engine coolant temperature sensor is not working properly
-Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
-Retarded spark timing
-The oxygen sensors in front and behind the converter are reporting too similar of readings
-Leaking fuel injector or high fuel pressure
-Cylinder misfire
-Oil contamination

Possible Solutions.
The easiest thing to do first is a visual check of things. Visually inspect the exhaust system for leaks, check the catalytic converter for dents, holes, severe discoloration, and check for a rattle inside. If any of those syptoms are there, the converter likely needs replacement. Then, visually inspect the downstream O2 sensor (behind the converter). Check for broken wires, obvious faults, etc. If all that checks out, you'll want to check the operation of the O2 sensor. To do that, you'll need access to a scan tool or oscilliscope. Check that the waveform is pretty steady. If the reading fluctuates then the sensor is likely bad and will need to be replaced.


If no problems found... Decarbonize the engine & add fuel system cleaner to the tank as an attempt to clean the cat.

 

Update....took the car to the dealer now as I found out there is a 8 year 80,000 mile on the cat converters and mine checked in at just over 76k miles and 6 years. After offering me a free diagnosis on my check engine light, they called back and it is indeed a cat converter gone bad. Good new it is being covered by the warranty, bad news is that the part is on national back order with no time frame on when they might have more. Hope they're still making em, my guess it is a pretty standard part that is on many other GM vehicles as well.