Parts or components should not be replaced with reference to only a U0939 DTC. The vehicle service manual should be consulted for more information on possible causes of the fault, along with required testing.
The U0939 check engine lamp is perhaps the most troubling of lights because it could mean so many different things, from you didn't screw the gas cap on tightly enough to look out for pistons flying through the hood and into the stratosphere. The easiest way to find out what this light is telling you is to hook your vehicle up to a scan tool. OBD 2 diagnostic tool looks a little like an oversized calculator and plugs into a communication port inside the car. After you instruct it to perform the scan, it speaks with your car's computers to find out exactly what's prompting the light to turn on.
Evaporative emissions systems test for a leak in the fuel tank and associated hoses. To perform the test, the computer pulls a vacuum and checks to see if the vacuum holds. Each car maker uses a different techniques to check, but most often it's some type of fuel tank pressure sensor.
Gasoline engines use spark plugs to cause an explosion of fuel within the cylinder. In a properly timed engine, this explosion occurs at the proper moment to send the piston to the bottom of the cylinder and provide power to the drive shaft. If the plug wires are out of sequence, the explosion occurs at the wrong time. The improper timing of the explosion sometimes pushes the cylinder the wrong direction or interferes with the turning of the crank. As a result, the engine stutters or backfires, if it runs at all.