Low mileage 2.3 L gives code P0420
 

Welcome to the forum

Post code and definition if you have already looked it up
P0420 Catalyst System Efficiency Below Threshold (Bank 1)

Also never Google any code number, look up a code on a FULL LIST , like here: OBD-II Trouble Codes : The Ranger Station

Computer usually has several codes it can chose from for any one system, seeing all of them can narrow down specific problem
This is Bank 1 Sensor 2 codes
P0420 Catalyst System Efficiency Below Threshold (Bank 1)
P0421 Warm Up Catalyst Efficiency Below Threshold (Bank 1)
P0422 Main Catalyst Efficiency Below Threshold (Bank 1)
P0423 Heated Catalyst Efficiency Below Threshold (Bank 1)
P0424 Heated Catalyst Temperature Below Threshold (Bank 1)

Bank 1 Sensor 2 is if you have a single exhaust system, dual exhaust would have a Bank 2 Sensor 2 as well, after the Cats

Age of Cat can matter with high miles, but low miles can be just as bad, Cat can rust out inside if it sat for longer periods.
So yes Cat could be failing, 11 years old with only 54k miles means it most likely sat for long periods or was never fully heated up while driving, i.e. short drives under 15minutes


Computer has these codes for failing downstream O2 sensor
P0136 02 Sensor Circuit Malfunction (Bank I Sensor 2)
P0137 02 Sensor Circuit Low Voltage (Bank I Sensor 2)
P0138 02 Sensor Circuit High Voltage (Bank I Sensor 2)
P0139 02 Sensor Circuit Slow Response (Bank 1 Sensor 2)
P0140 02 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)
P0141 02 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)

And since these are not present it would seem that the sensor is not the issue
So time for a new Cat

Cat converters working or not don't effect engine performance one way or the other, they are there to clean the exhaust for less pollution, so neutral as far as engine operation.
If they break apart inside they can clog the exhaust which will effect performance

Exhaust bolts rust, so I wouldn't call any exhaust work easy, until after I was done and then maybe say...."well that was easy", lol

If parts store is a ways off, or this is your only vehicle, look at the clamps, hangers, nuts and bolts used and get those along with the Cat, you can take them back if not needed.
Have a grinder or metal cutting saw just in case.

If you have emissions testing in your area there can be "rules" for installing new Cats, they need to have certain certificates that must be shown to tester, and in some states DIY installation of Cats is not allowed
One way around it is to "dirty" the new Cat up so it doesn't look new when they do the visual inspection

Thanks for the advice! The CEL went off and has stayed off for a day so far. Anyone else had this problem? I wonder how frequently the code will come up. I think MD has Federal/EPA emissions standards, so when the time comes, I can put an aftermarket cat on and keep everything legit. I've also never had someone check the exhaust visually during an inspection...