P0420 error

My 1996 5.0L Silverado has 153,000 miles. The catalytic converter was replaced at 105,000 miles. My service engine light is on and my OBD II scanner shows a P0420 error (converter inefficency). The service shop scanned the computer with their unit and told me that the error is being caused by the Bank 1 injector running to lean and that its only a matter of time before it stops functioning.

My concern is that I have a 50,000 mile warranty on the converter. Does their diagnosis sound reasonable

Thanks for your help.