Most EMS's fitted to production vehicles are not programmable, that is to say that the maps within the EMS which determine the fuelling and ignition settings are fixed and cannot be changed by the owner.  This makes good sense from a manufacturer's point of view since the engine then runs within the permitted parameters, which keeps the engine emissions and economy within known limits.

There is a prosperous market for ‘chip-tuning’ where the chip or information containing the maps is replaced by another which has revised map settings providing better performance from the engine.  The gains to be had here are usually fairly small except with turbo-charged engines where the EMS controls the boost.  Chip tuning or re-flashes on these engines can yield quite large increases in engine power.  Some manufacturers go to great lengths to stop after-market tuners from decoding the maps within their EMS - with varying degrees of success.

All after-market EMS are programmable since they have to be fitted to a variety of different engine installations in a variety of states of tune.  If the map values could not be changed then the EMS would be useless for after-market applications.  Some manufacturers of these systems discourage home mapping and will only allow authorised dealers to undertake the mapping.

For clarity's sake we will examine each of the two sub-systems within an EMS separately.  In practice there is a great deal of interaction between the two, as both systems utilise information from the various engine sensors.