Test sequence

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Disconnect glow plug connectors from glow plugs.
Connect voltage test lamp wire to B+ using adapter clips from connector test kit.
Attach test lead of voltage test lamp to each glow plug in succession.

Fig 1: Attaching Test Lead Of Voltage Test Lamp To Each Glow Plug
Courtesy of VOLKSWAGEN OF AMERICA, INC.

LED lights up: Glow plug OK.

LED does not light up: Replace glow plug

Remove and install glow plugs using 3220 flex wrench
Tightening torque: 15 Nm.