[JettaRed]

The check valve is a simple ball and spring type. I had gotten debris in mine (probably from stuff in the washer tank over the years) and it was not sealing properly. I had rough idle, misfires, and Oxygen Sensor DTCs. I used an airgun to blow through the check valve (after removing it) and then took it apart (easy to do, but didn't need to) to make sure there was nothing to lodge between the ball and valve seat.

Now, everything is running great. Check your check valve.