OK, here is a weird problem with my 1996 Hyundai Elantra. Recently, my car was losing power and rather sluggish. So I tried to change the fuel filter. But I had the wrong one. So I put it back together and forgot to hook up the Mass Air Flow sensor. Needless to say it didn’t run well, but I needed to get to work. So I drove it there and back (about 100 miles in total). The next day I took off to get it repaired and inspected. Took it to a muffler place and had a new catalytic converter put on. Then to the mechanic and got it inspected, he discovered the unplugged MAF sensor. He took it off and cleaned it and plugged it back in and it seemed to be running better.
So I took it back work the next day and it had the same problem. On the way home it threw a code (P0141 O2 sensor heater resistance problem). So back to the mechanic the next day. He replaced the O2 sensor.
It did not fix the problem. So I got the right fuel filter and a new PCV valve. And that seemed to fix it. Went to work and came home no issues.
Until this morning… I filled up the tank and the issue, which is mainly that it hesitates on acceleration especially at low speeds.