1990 Nissan Sentra 2d - "check engine" light collaborating with gas tank

My car is in excellent condition all the way around, in general. The check engine light has been on since I purchased it. I’ve started to notice that it goes off when the gas tank is practically empty. It goes back on when I fill up, and then stays on until well below 1/8 of a tank. Someone told me that it could be a gas cap sensor, and this car does have a new gas cap (one that needs a key to open). Any ideas out there?

One of the best ways to find out why the CEL is ‘on’ is to scan the computer for codes. The actual codes would help you and us focus on the likely problems. So come on back with them.

I can’t find anywhere that can scan this old of a car for free in my area. The auto stores are telling me they can only scan 1996 and newer. That’s why I’m posting. Trying to find out if this sounds like anything before I start investing.

Here’s the DIY method for getting the code(s)

Alright. There’s red and green LED lights. They both flash at the same time. Twice slowly. Three times fast. Four times fast. Five times fast. Once slowly. I have the manual for this car, and it doesn’t mention any of this! Thanks for your help! What do you think?