Bosch is fine... you need to test the resistance of all 4 plugs to find the bad one. Set your amp-meter (assunimg youhave one) on it's lowest setting and check each plug. They should all be within .5 of each other.greenflyer said:How do i find out which glow plug is which? I had my TDI scanned and they said it was the # 1 cylinder glow plug that needed replacing. And what kind of glow plug is it? or can i just replace it with any bosch glow plug?