Most likely the speed sensor. It's located on the back top side of the transmission. Look for the hump on the trans, and it'll be to the right side, it's the 3 pin plug. Disconnect it, unbolt the bolt (torques) holding it in, and pull the sensor out.
The idle speed sensor tells the onboard computer how fast your engine is idling. The location of the idle speed sensor on the VW Jetta is on the actual throttle body.
The 2004 Volkswagen Jetta has a 5-speed manual.
The 2006 Volkswagen Jetta has a 5-speed manual.
The 2001 Volkswagen Jetta has a 5-speed manual.
The 2008 Volkswagen Jetta has a 5-speed manual.
The 2013 Volkswagen Jetta has a 5-speed manual.
The 2009 Volkswagen Jetta has a 5-speed manual.
The 2003 Volkswagen Jetta has a 5-speed manual.
The 2007 Volkswagen Jetta has a 5-speed manual.
The 2013 Volkswagen Jetta-GLI has a 6-speed manual.
The 2012 Volkswagen Jetta-Sportwagen has a 5-speed manual.
The 2014 Volkswagen Jetta-GLI has a 6-speed manual.