CONSULTATION

Hello friends, I need information about the preheating system of the 2.0 16V Ecotec engine. (My car is an Astra GLS, equipped with that engine). The reason for this inquiry is that two days ago, it started having trouble starting. I located the glow plugs, but I don't want to damage anything until I know how to remove and test them, as well as the timer. I would greatly appreciate this information. Regards.