I would like to know which operations or functions does the car do when I turn the key to On position (the key position before starting the engine) in gas engine and in diesel engine. For example, in a diesel engine I know that the engine got warm.

1 Answer 1


This varies a lot by model, era etc., but a general overview:

The first thing that happens is the ancilliary electrical systems are powered:

  • Lights
  • Radio (often the radio will be permanently wired, but this varies)
  • Windscreen wipers
  • Dashboard gauges
  • Fans
  • Anything that runs off switched ignition

but also the Engine control unit, or ECU, which is what controls the engine (spark timing, fuel flow, airflow etc) so that it is ready to start the engine

Many of these are then switched off when the starter motor is engaged, just to reduce the load on the battery, as the starter motor is the largest current draw in your entire car, but once the engine is running and the key is released it moves back to the on position.

In a petrol engine there is no need to preheat the cylinders like you have in a diesel.

  • 2
    Also, the ON position primes the fuel pump. Commented Mar 31, 2014 at 10:20
  • It also does the safety checks, like anti-lock brakes and airbags. Commented Apr 1, 2014 at 10:37

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .