I really don't have a big problem. It's just that I went to Autozone and had this guy to tell me why my check engine light was on and he told me that my Oxygen sensor was going bad. He told me that I could change to 89 fuel to see if it would help. Would this possibly solve the problem? Is it safe to use 89 fuel in a 2002 Mitsubishi Lancer?

Actually yes. I had the light come on in my vehicle so I went to the local car parts place and they did a free code check.
I was told to take off the gas cap and put it back on.
We restarted the vehicle and the light did not come back on.
The older the gas cap gets the gasket wears out. I however had just not seated mine on right after I had gassed up. lol
Does not always have to cost $100's for a fix.

Who told you these things? Autozone? Those guys don't know their kester from a hold in the ground about electronic engine control systems.. This is a very expensive part, have a qualified technician diagnose the problem and replace the correct part or make the right repair.

Make no repairs until a proper diagnostic is run on the cause of the check engine light by a factory trained technician.

I'm sorry,MIke! I don't have any manual for your truck.Check engine light codes come with an explanation on my reader,which really helps.
The last guy I told to get a shop manual got mad at me,but I'd really recommend it.don-ohio

from autozone.com

If they did not give you a trouble code than find someplace that will. It is really uncommon to have knock sensors go bad. It is more common for a mechanical failure to cause the knock sensor to become more active than it should causing the knock sensor code.

Does the engine make any noise when it is running?

The best thing you could do is find out specifically what the code is that is causing the check engine light to come on. Then research that code for a solution.

BTW, we get a lot of cars in our shop that have been to autoszone and have had no resolution to there problem. I agree with emission wiz, it takes years of experience under the hood to be good at figuring this stuff out. You can't hand a guy a scan tool and expect him to solve problems. That guy with the scan tool at autozone more than likely, barely has more automotive experience than you do kgirman! :)

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

Check the relay for the fan. My fan wasn't coming on and went to advance auto they told me to check that first it's $37 part u have to take the grill, the head light, and bumper off its just below the headlight in a spot just big enough for your hand to fit into. I had a guy telling me its the computer Not just the relay fan switch.

Try disconnecting the battery for about 20 minutes or so. This could reset the light. If it does not, call the dealer, service department and ask them to tell you how to reset the light.
I hope this helps you.

Take it to a parts store like Autozone or Advance. Also any maintenance shop. They can run a check on the engine light to get a code. The code will tell you what the computer thinks is the problem. My Toyota, it was the oxygen sensor, which looks like a spark plug and is attached to the exhaust system. I would also just look around and see if there is something else that is not connected. Could be a wire connection or a hose, a loose vacuum hose can make a big difference in the performance.

First thing to remember autozone people ar not experts in the automative diagnostic world, if they were they would be earning 50 - 75,ooo per year as a technician, not 25,000 as a desk jockey. You need to go to a mechanic that has a computer to hookup to your car and tell you what the check engine light is on for. then you can fix the exact problem instead of poking around in the dark at the problem until you get lucky and have it fixed for a week or two,