On the way home from a 3 hr trip, my car began to idle roughly. Sputtering and RPMs dropping low when I stopped. Soon after, my check engine light (CEL) came on. Car was getting worse so I took it to the parts store and had them read the codes. I got 4: P0139 O2 sensor bank 1 sensor 2 CKT slow response, P0302 Cylinder 2 misfire, P0442 and P0456 for emissions leaks. Not too worried about the last two, since they were probably set of by my gas cap.
Looked up causes for P0302. Main cause seemed to be spark plugs, wires and coil. Checked all three out and now I'm out of ideas. I'm nervous about taking it to the shop since I don't know any good mechanics around here.
Anybody have the same or similar issue?

 

More information is needed. Also, you left out the year and mileage of your car.

How did you check the wires, plugs, and coil pack? The plugs should be inspected for physical damage/issues. The wires should be inspected for carbon tracks and arcs along the boot and wire surface. They can also be checked for proper resistance. The coil pack can be checked for the proper resistance between the windings. A shop manual will give you much of this information. Is there any arcing around the wire area when the engine is running?

Did you actually leave the gas cap off? The evap leaks could be from any number of things.. rusty fill pipe, cracked and worn hoses.

 

Oops. 2005 Elantra GLS. 106619 miles.
Checked all for physical damage/issues. Plugs, wires and all were clean. Tested each spark plug the cheap way (plugged them in and hit metal with it). Yeah, I know it's not safe but it was late and was the only way I knew how to check the spark without tools/equipment. Also checked everything by taking the power off each sparkplug one at a time. Engine sounded the same regardless of whatever plug cap was off.
Not a car expert, but with no tools and no professional, did the best I could at the time.
No arcing either.

 

If you can get another wire from another set, try swapping out the wire on the #2 cylinder. Pull out the plug and see what it looks like, compare it to the others. The coil pack can be checked with an ohm meter. If everything checks out, perhaps that cylinder is not getting fuel. Using a piece of tubing, stick one end to the injector and the other to your ear while the car is running. Does it sound different that the other injectors? It should be making a rapid clicking sound.

The 02 sensor reading may indicate and exhaust leak somewhere. Any section of flex pipe is a likely culprit.

 

Plugs and wires each 60,000...

With engine at idle, take a can of carb and spritz lightly (not a soaking power wash of solvent) the seam for the intake to cylinder head mate surface... brooken intake gasket is about the oldest in the Elantra-Tiburon 2.0 handbook. Carb hits the vacuum leak and gets drawn into intake, you'll know it.