Service Interval Indicator (SIA IV)
Beginning with the E46, the new SIA IV service interval indicator system is now used. The new system now longer uses the colored LED's as used in the past, but rather a numeric display of mileage remaining to the next service.
With this system, the actual mileage remaining until the next service will be displayed when the ignition is turned on. The service mileage will be displayed in the odometer display for approximately 5 seconds.
The text "Oil Service" or "Inspection" will also illuminate to show which service is due. A minus (-) symbol before the mileage display indicates that a service is past due.
The calculation process for determining the service interval is similar to SIA III. Like the SIA III system, the SIA IV system calculates service intervals based on fuel used.
A reference value of fuel is stored in the instrument cluster during the coding process, this value is in gallons and is model dependent. As the vehicle is driven, the instrument cluster monitors the injector on time ("ti") signal and deducts the amount of fuel used against the reference value.
In other words, the injector on time is an indicator of how the vehicle is driven. For example, a large number of short trips would consume more fuel and therefore the average injector on time would be greater. This increased fuel consumption would reduce the service interval mileage because the average injector on-time would be higher and would deduct gallons (theoretical) from the stored value (Coded Inspection Limit) at a faster rate.