I have a 2003 Hyundai Elantra. Its has a cylinder 2 misfire an then the computer is showing a oxygen sensor slow response. 2 months ago I had mutiple misfires. I changed plugs still missfireing. So I changed all 4 fuel injectors. There was no noticeable misfire but the computer still read cylinder 2 misfire I didnt think anything of it because I couldn't tell it was misfiring. So I figured it was a code stuck on the computer. Now 2 months later its back to misfiring like it was 2 months ago. Iv checked the plugs and wires. I swapped injectors around to see if that was it.still misfiring on the same cylinder. I sprayed dw40 all around the intake manifold gasket. No rev in the motor so leak. Im trying to do whatever diagnostics I can at home. Im low on money at the moment. Any advice can help thank. Also oil has been changed every 3000 miled im a firm believer in that.plus when it first started misfiring 2 months ago if I turned the ac on the performance of the car dropped like acceleration an stuff. After I fixed the misfiring 2 months ago it still did it. Idk why its doing that.