Test sequence

Pull glow plug connector off glow plug.
Connect diode test lamp wire using auxiliary clips from auxiliary cable set to battery positive (+).

Fig 1: Placing Diode Test Lamp Probe On Glow Plug
Courtesy of VOLKSWAGEN OF AMERICA, INC.
Place diode test lamp probe on each glow plug one after the other.
Diode lights up: glow plug OK.

Diode does not light up: replace glow plug.
Remove and install glow plugs with Hinged socket 3220.
Tightening torque: 15 Nm.