When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
It's actually not a glowplug, at least in the same sense. Just a heater coil. Referred to by many/most here as the AIH(Air Intake Heater)
Here are the requirements for it to activate
Ambient temperature must be below 32°F
Engine oil temperature (EOT) must be below 131°F
Vehicle power must remain between 11.5 and 15.0 volts
Parking brake must be applied
The transmission must be in Park or Neutral
Glow plugs must be off