What's it mean when SES light doesn't come on, but a code shows up on a code reader?

Been chasing p2a00, checked for intake and exhaust leaks + changed b1s1 to oem sensor.

Thanks!

 

Usually because it's pending

 

Before I replaced the o2 and fixed an intake leak, the code would come on at 50 miles exactly. When i clear it, the code doesnt show up untill the first 10 miles or so. Light still wont come on, just shows it in the code reader. Then last time at 100 miles the code popped back up. Havent put 100 miles on it yet to see what it does after fixing that intake leak

 

Just be aware that not every OBDII DTC triggers a CEL. Some are "minor enough" to not light the panic button. But they do get stored, hence a good thing to check from time to time with or without a CEL thrown.