In case your car has the 13-button OBC and that moron of a previous owner lost the owner's manual, here is the explanation for each function:
If the power supply has been disconnected, then the OBC resets to German and metric units. To set it to the appropriate units of measurement, press AVG MPH. Then, using a ballpoint pen, press the changeover contact button once (the little button located on the lower right corner). Press AVG MPH once more for American units, twice for Imperial (M/G), and thrice for metric (L/100). Press the changeover contact button again to set.
Tells both the time and date. If the display is showing incorrect units (e.g. 24 hour clock or reads "UHR"), read the paragraph above to set it. To set the time, press the changeover contact and use the 1000/100 buttons to set the hour and the 10/1 buttons to correct the minutes. Press the changeover contact button again to set. A dot will appear between the hours and minutes to indicate that the clock in running. To set the date, press HR-DATE again and repeat the process. The OBC does not take leap years or daylight savings into consideration.
Displays the current average speed. Press SET-RES anytime to reset the calculation.
Works as a speed limit warning. To set, press M MPH once, then using the 100/10/1 buttons, enter the maximum speed you desire, then press M MPH again to set. If the entered speed limit is exceeded, then the LED will flash and the door gong will sound as a warning. To cancel the feature, press M MPH so the red light is no longer illuminated.
Displays the current average fuel consumption. Press SET-RES anytime to reset the calculation.
Displays the estimated distance until the fuel tank is empty. When the range falls below 9 miles (13 km), it will flash to alert the driver to refuel the vehicle. A "+" sign before the display means there is a greater range than indicated as a result of overfilling the gas tank. The OBC does not calculate the addition of fuel unless at least 1.3 US gallons (5 liters) of fuel is added.
Works as a stopwatch to measure times during travel, standstill, and parking. The display shows the running time or the time at which the timer was last stopped. Press SET-RES to start the timer and again to stop it. The maximum time it can measure is 99 hours and 59 minutes.
Displays exterior temperature. If the temperature falls below 37 degrees F (3 degrees C), the door gong sounds to alert the driver of possible icy road conditions and the display will flash for 8 seconds. The gong will sound again when the temperature rises above 43 degrees F (6 degrees C).
Works as a vehicle immobilizer for anti-theft protection. It requires the entering of a code to start the car. If an improper code is entered (or none at all) the starter motor will turn but the engine will not start. If three incorrect codes are entered in succession, or if three attempts are made to start the engine, the horn will sound for 30 seconds. The code must be entered and set at ignition position 1.
To set the CODE feature for the first time, turn the ignition to 1 and press CODE. Using the 1000/100/10/1 buttons, enter any number between 0000 and 9999. (If you make a mistake, turn the ignition to position 2 to cancel.) REMEMBER THE NUMBER! Then turn the ignition to 0 to store the code and activate the system. To start the car or to cancel the feature, turn the ignition to either 1 or 2 and input the correct code. The feature will then be cancelled by starting the engine or by pressing SET-RES; if neither of the two are done then the code will be retained in memory.
Used to reset AVG MPH and AVG MPG and to start/stop the stopwatch (TIMER). If the CODE feature is used then the actual time will appear when the correct code is entered and SET-RES is pressed.