I have a 2002 Elantra GLS that has been acting up lately. About 6 weeks ago the check engine light came on after the car started having slight engine problems. At a stop light when you would start to drive the engine would hesitate to a second or two and not rev, but would then go back to normal and rev just fine. This would only happen a few times a week. Lately it has been doing it a bit more, so I went to Autozone to run an OBD-II check which came up with the P0130 code "02 Sensor Circuit Malfunction (Bank I Sensor 1)" which is the O2 sensor on the exhaust manifold. After doing a bit of research on various sites I have come to the conclusion that it is one of the following things; the o2 sensor itself, plugs and/or wires, fuel pump or MAF. I changed the plugs the other day along with cleaning the electrical connection to the O2 sensor and plug wires to see if it was a bad connection. After that the car ran just fine, but with the check engine light still illuminated. The day after I did that I took the car out for a drive which ran just fine at first, but then the problem returned even worse than before. It got to the point where the engine would barely rev at all below 3500 RPM, but once you reached 3500 RPM the engine would rev to redline just fine. So when I came to a stop light I had to play with the throttle and clutch just to get rolling because the engine was making no power in the lower RPM range. Later my dad tried to take the car for a spin and it wouldn't even rev at all no matter how much throttle was given. It would idle just fine, but nothing more. This morning I was going to try unplugging the MAF to see if that would allow the engine to actually rev above idle, but when I took car out before unplugging the MAF the car ran perfectly fine. I drove around for a while allowing the engine to get to operating temperature and driving throughout the RPM range and the engine ran normal. This has led to much confusion as to what the problem could be. There is no trouble code for the MAF, but it has the symptoms of being a faulty MAF. Also I have never heard of a worn out of faulty O2 sensor affecting the performance of an engine so badly. From what I have read the signs of a faulty O2 sensor is bad emissions and higher fuel consumption. And it seems like the problem is to sporadic to be a fuel pump. I will be starting the trial and error process of changing parts one at a time to see if that helps, but I wanted to see if anyone had any similar experiences or would have a better idea of what the cause could be. Thanks a bunch!

Search around for exhaust flex pipe problems on the forum. I seem to recall the problems being similar to this. Might be something to check. Good Luck!

Have you used an aftermarket air intake or a oiled drop in filter? If not check to see if the MAF is dirty anyway.