Modern electrical systems means it is done before you even think about pressing the button. It could be as soon as you unlock the doors the plugs are heated.

Modern common rail diesels rarely use the glow plugs for cold start unless the outside temp is well below 5 degrees C . They are more likely to be activated during the dpf regen process .

Glow plugs are mainly used nowadays to reduce post - cold start diesel exhaust smoke rather than simply as an aid to actual starting, They can remain energised and heating for a short while after a cold start, dependant on air and coolant temperature, despite the fact that the preheat light is not illuminated.

Witnessed by how MOH starts either of our diesels even if it's zero degrees or lower, she just jumps in and turns the key and starts them like a petrol engine, they start within 2 to 3 revs.

Despite being told to wait for the light to go out etc :-)