I have an 06 cobalt 2.2. The po420 code came on a couple days ago. The cat and both sensors were replaced about 3 months ago by a licensed exhaust specialist/mechanic. What else could be the problem?
Which sensors were replaced? If the downstream O2 sensor wasn't, it may need to be. If that's not the problem, it's probably as simple as it looks: the catalytic converter isn't performing up to snuff. This could mean the new cat that's in there is a low-quality unit that isn't performing to spec, or the it's been ruined like old one was. Was any additional work done to the car when you had it replaced?
Catalytic converters don't go bad on their own. If everything upstream of them is working as it should be, they should outlast the car. However, if something else is happening like the mixture is too rich (too much fuel), they can quickly get destroyed. I don't know how soon, exactly, so I can't say if 3 months is surprising or not.
I'd bring it to a shop and have them give it a good look-over and see what they say. If you're interested in doing it yourself, start looking for the causes of things like smoke out the tailpipe, rough idles, hard starts...anything out of the ordinary.
The fuel system, combustion chambers and valves are dirty.
Start by changing the motor oil and filter with a top synthetic oil like Mobil 1. Substitute one quart of oil with Marvel Mystery Oil (or automatic transmission fluid).
If the car has a PCV valve and EGR valve, make sure both have been replaced and the tubing cleaned with carburetor cleaner. Replace the air filter and spark plugs. Check the intake manifold and vacuum hoses for air leaks.
Add Marvel Mystery Oil (or automatic transmission fluid) to your gas tank at fill-up at a treat rate of 1 oz. per gallon. Drive the car immediately in stop-and-go traffic for 10 minutes, shut off engine for 10 minutes, then drive on the highway at 70 MPH for 10 minutes. Repeat this cycle multiple times, for at least an hour, and allow the mixture to penetrate at least two days.
When your tank drops to 10 gallons of gas, add one can (15 oz.) of Berryman B12 and one can catalytic converter cleaner. Immediately drive the car on the highway for 30-60 minutes, then "floor" the car at wide-open-throttle (WOT) from 50-80 MPH. Repeat the full throttle blasts at least six times, but up to 30 times if the cat. & sensors are very dirty. Be sure the cooling system and transmission are flushed and working properly beforehand. Turn the heater and fan on maximum, with windows open, when performing the WOT blasts.
The Berryman's and cat. cleaner evaporate out of the gas tank after about 24 hours, so be sure to run the car hard right away.
When the problems are resolved and you refill the tank, add Marvel Mystery Oil at a treat rate of 4 oz. for every 10 gallons of gas. It will continue to clean deposits. Every fourth fill-up, add one can of Berryman's and drive at highway speeds.