where is the engine control module located on a 2007 chevy silverado 5.3 liter

P0420 is bank one catalyst inefficiency. This is far more likely to be related to a catalytic converter that is not working the way it should. ECM wiring will not be a factor. Naturally, testing should be performed using a diagnostic tool. The oxygen sensor before and after the bank one catalytic converter should be monitored for change. If they stay the same, the catalytic converter is not working as it should, and must be replaced pursuant to the P0420 code.