Question

2003 Chevrolet K2500, the O2 system monitor refuses to check in after a thousand miles. It won't pass

Answer 1

When the 02 monitor will not set, on the scan tool data there will be a listing for " Fuel System"  it should list both sides of the Bank if it says "OPEN" the ECM does not see the 02 sensor, The cause here can be if the ECM was reprogrammed. with a performance reprogram. This is where the history of the vehicle is needed. If the Fuel System reads "Closed" listed for both side of the Bank  the ECM does see the 02 sensor.