Six months ago, I installed new spark plugs, ignition wires and coils on my 2003 Silverado 5.3l. Now I am getting the same error code as before doing that. What else could give the same error code?

 

any other dtc setting? post all the codes.

if its only misfiring at idle....intake manifold gaskets. if its misfiring at wot....exhaust restriction, low fuel pressure.

you need to view live misfire data to see which cylinders are effected. knowing fuel trim data at idle and 2500 rpm would help in diagnosis also.

 

Hard to get what exact codes you are showing but if you have P0300 and have new spark plugs, coils and wiring I’d test the spark on a ground of the block and ensure you have the proper spark, if all spark I would look into the injectors and see if you may have one clogged.