2002 gmc safari cargo van 4.L engine

I recently replaced the distributor cap, rotor button in cap as well as changed the coil out. She started fine but after it got hot after initial start it would have a lot of trouble starting even with much cranking. Now it will not start at all. I checked the codes and it comes up P0756 and P0420. I even took off the positive cable from battery in attempt to see if computer resets but I still can’t get ole Betsy to come back to life. Any ideas to try this time?. Thanks. Abert