Trying to figure out different typical reasons why the check engine/service engine now light would be on?

 

To start, get the codes from the system and see where it leads.

 

There are dozens of reasons. Don't waste your time chasing dead ends. All you need is a paperclip to get the fault.
flash codes: https://repairpal.com/gm-obd-i-code-chart

 

Thanks. I've been trying to pull codes that way for a couple of days and only giving me the status (12). Finally on this colder morning, it finally popped up a code for the ignition. I thought it was the gas cap or O2 sensor at first. Hopefully after fixing that the light goes away.

 

Code 12 will always be the first code displayed, regardless if any other codes are stored. The codes also repeat 3 times before it blinks out the next code.

As an example - if you have codes 15 & 23 stored, the codes will flash the SES/CEL as follows:

“1” represents a flash
“-“ represents a short pause
”- -“ represents a long pause

1 - 11 - - 1 - 11 - - 1 - 11 (code 12) - - 1 - 11111 - - 1 - 11111 - - 1 - 11111 (code 15) - - 11 - 111 - - 11 - 111 - - 11 - 111 (code 23) - -

Then it will start flashing out code 12 again and the sequence repeats.