For late OBD-II vehicles such as 2000 model year, the plugged cat converters will throw codes. I've been diagnosed a 2005 Chevy truck w/5.3L V8 that a customer complaint no power and rough running, codes: P0300 random misfire detected, P0171, P0172, P0174 and P0175 lean/rich. Fuel pressure ok, oil level ok, I noticed fuel trim on Bank 1 (Driver side), were erratic, look like valves on the B1, malfunction. I took it for road test with my Modis Ultra scanner to observe the fuel trim, this vehicle won't go as faster as I accelerate, caught fuel trims in screen of my scanner that B1 was not match to B2 (passenger side). B2 fuel trim normal, return to the station, decide to lean over the cat converter, test it with back pressure gauge, saw my gauge show over 15 psi when snap gas pedal, obviously the cat converter on B1 was bad plugged. Sublet this truck to the muffler shop to replace a new cat. Truck runs a great, no codes return.Plugged cat dosn't always throw a code. Exact symptoms happened to my brothers truck so we put a fuel pump in it and it ended up being the cat. Egg on my face for not checking.
For older OBD-I and early OBD-II vehicles will not throw codes when it's plugged cat converters.