I reserve the right to be wrong, but I belive if the condition which set the code is still present the light will remain on. If it just erased the code people could pass emissions with out fixing the problem. My best guess is the code he got was for the charcoal canister vent control valve which open and closes to test the system. If the valve gets stuck and causes the code and then frees up the code would clear.

 

The light goes out -if- the problem cause is intermittent - and vanishes. Pls refer to the FSM about the detailed preconditions for this to happen; seek via link(s) - clic below.

 

Ok sorry I missed the "if" and just saw "ECU shuts the light off after programmed period (time & drivin profile)."

 

-If- word gives leeway for hillybillies like me...

Another good one is 'why'. Repeat 7times to a scientist and big bang collapses like warmup hoax industry (where this enviro -milkingfeature is based ; thinkabout the billion$ global revenue keeping ppl paying for these hypocrite earthsaving systems )