I have a 2006 elantra with 89000 miles. The check engine light came on and the codes were p0301, p0304 and something about the o2 sensor. I had the spark plugs and wires, ignition coil and the o2 sensor replaced. Had a mechanic look at it, he said he couldn't find anything wrong with the car. He erased pending codes, told me to drive it over the weekend. After 30 miles the engine light came on again and this time the codes are p0301, p0302, and p0300. At times when I start the car, it will idle rough (shake pretty hard) for a few seconds and then seems fine.Can someone give me an idea of what could be wrong with the car. I don't want to keep replacing parts. It's getting expensive and being a female and not knowing anything about vehicles, I dont want to be taken advantage of.

 

Those are misfire codes. P0300 is a general code, P0301 is a misfire on cylinder 1, P0304 is a misfire on cylinder 4. If the mechanic can't find the problem go to a better mechanic. The one you want to doesn't know what he's doing.