Google that code on your computer.It will tell ya.

P0420 Catalyst System Efficiency Below Threshold Bank 1 U110C I think it is manufacturer specific code, you will need to consult your car's manual. In most cases, This is a communication failure between the BCM and the PCM on the fuel level bus line. There is either a open in circuit between the BCM and PCM or faulty BCM.

U codes are communication codes. A bus line is down in the vehicle. You need to determine which computer is down. The P0420 is not likely related to the communication code, as all the input for the code are ECM related. In some cases you may see U codes where communication issues may be relevant. One example might be communication from a TCM and ABS module, as these may be related as virtual systems may be shared between the 2. For example, the vehicle might use a WSS sensor for the vehicle speed input. Or in the case of Kia, a bad RF WSS or communication between the ABS may create readiness issues as they like to use the RF WSS for rough road and crankshaft variation strategy, thus rendering the misfire monitor as never complete. Check the fore and aft O2s, but odds are you need a cat.

thanks sandt38.