U0D00 trouble code??

Hi, I'm really hoping you can help... last week my 2001 Hyundai Santa Fe started to shake while driving. When I stop at a light it feels like it's going to stall but doesn't. My check engine light came on as soon as the shaking started. I took it to have the trouble code read and it came back with a 4th cylinder misfiring code, so I had the 4th cyl spark plug changed, as well as the coil pack. The light came back on almost immediately and it's still misbehaving. Tonight I had the code read again - once again the 4th cyl misfiring, and also U0D00 came up. I can't find any info on this and hope you can help! Some more info - antifreeze in the reservoir is "disappearing". Last month I got a new radiator and hoses so I don't suspect a leak anywhere. Also I'm completely broke and can't afford a major repair or a new car, so really hoping this is a simple fix! Thanks in advance!!