Glow Plug
WARNING: This page is about a different car, the 2010 Volvo V70, 2009 Volvo V70, and 2008 Volvo V70. However, it is still accessible from the selected car via links, so may be relevant.
The glow plug is positioned so that part of it is in the combustion chamber. It has a wire that starts to glow when it receives current from the control module. The glow plug is activated by the control module and then ignites the fuel/air mixture in the combustion chamber.
The control module also uses the glow plug to register whether there is a flame in the combustion chamber. The signal from the glow plug is used to switch off the heater, etc.
There are diagnostics for the electronics of the glow plug.