Code P0301, check engine light keeps turning on?

Hello. Make sure your Fuel Injector is actually firing. Here is a quick guide to check its activity with a test light. If you have power on one wire of the injector and a pulse on the other wire (you'll see in the guide), and you're sure you are getting spark to that cylinder, do a compression test next. The guides are really easy to follow. On a vehicle this old, it might have a compression issue on that cylinder.
The black wire with a white stripe will be the 12v power wire on the Fuel Injector and the other wire will have a dim flashing when the Test light is hooked to Battery Negative while its running if the Injector is firing.

https://www.2carpros.com/articles/how-to-test-a-fuel-injector

https://www.2carpros.com/articles/how-to-test-engine-compression

Were you able to do the test light check on the Fuel Injector? If its a bit unclear, the computer will turn the ground side of the Injector on and off (pulses) extremely fast. So, if you hook the test light to Battery positive, the test light will flash because its acting as the Fuel injector should act. The test will flash dimly because the computer is now grounding the test light and pulsing it. This test will work with the Injector plugged in or unplugged. Back probing the Injector harness plug while it's still plugged in is the better way to do the test because the fuel injector will put more of an electrical load on the circuit. If you have 12 volts on one wire and a dim flash of the other wire, the fuel injector is being controlled correctly. And doing a compression test would be next.
If you have a scan tool and that's how you are getting the codes, when the p0301 sets again, there should also be an option on the scan tool that says "Freezeframe Data". This is the live data that was happening when the code set. It can be very helpful in determining what may be causing the code.
Some scan tools have options that are semi hidden sometimes. So, taking the time to see what else the scan tool can do might help as well. Such as Live Data as the vehicle is running.
Some compression issues will not show up on a static compression test, (no running) A sticking intake or exhaust valve may not show up, so doing the compression test at different times, such as a hot engine versus a cold engine. And when doing the compression test, compare the number one cylinder with other cylinders to see if the levels are different. Just disable the fuel injectors when doing the test, unplug them if it's easiest.
If the cylinder comes up low on compression, a small shot of oil in the cylinder will seal up the piston rings and you can determine if it's a ring problem or a valve problem.