Hyundai Repair: 2005 Hyundai Elantra, 2005 hyundai elantra, hyundai elantra

Question
Intermittent engine light.  Currently not on.  Car runs fine.  Checked for error codes and it showed P2251 and P0304.  Any suggestions on what the problem is?  Solutions?

Thanks!

Answer
Hello Tom,
P2251 L.O2 reference circuit(-) open-Sensor 1. This code is showing a problem on the reference voltage of the upper O2 sensor. It might be sensor or the wiring. P0304 Cyl.number 4 Misfire detected. This code is showing 4th.cylinder is or has been missing. Second code might be set by first code.O2 sensor problem needs to be corrected first.