How does your Altima 2011 act when your catalytic converter is bad

You probably won't notice a difference in drivability unless its clogged up. In that case you'd experience low power and most likely a rough idle from all the exhaust back pressure. All the car is doing is measuring how clean the exhaust is after the cat. If it falls outside of a predetermined threshold it will tell you that you require a replacement. Unfortunately they are quite expensive as they contain precious metals. Have the O2 sensors tested before you go ahead with a new cat. It would be a shame to spend all that money to find out it was a sensor all along. Best of luck