Check engine light stays on after check by scan tool

Just checking the codes will not turn off the light. Neither will clearing the codes if there is still an existing problem. So say it's a P0300 code, that is a random multiple misfire and it is a continuously monitored item Type A. If whatever is triggering that code is still happening it will come back as soon as the engine is started. Same thing with many other codes. Then you have a type B code, this is a code that sets after two failures of the test that sets the code. So you clear the code and drive the truck and it fails the test but no light comes on. The next day you drive it and it fails the same test and now the light comes on.