The "Malfunction Indicator Light" ( MIL) will be illuminated under the following conditions

• Upon the completion of the next consecutive driving cycle where the previously faulted system is monitored again and the emissions relevant fault is again present.
• Immediately if a "Catalyst Damaging" fault occurs (Misfire Detection).

  The illumination of the light is performed in accordance with the Federal Test Procedure (FTP) which requires the lamp to be illuminated when:

  Fig 1: Identifying Malfunction Indicator Light

  Courtesy of BMW OF NORTH AMERICA, INC.
• A malfunction of a component that can affect the emission performance of the vehicle occurs and causes emissions to exceed 1.5 times the standards required by the (FTP).
• Manufacturer-defined specifications are exceeded.
• An implausible input signal is generated.
• Catalyst deterioration causes HC-emissions to exceed a limit equivalent to 1.5 times the standard (FTP).
• Misfire faults occur.
• A leak is detected in the evaporative system, or "purging" is defective.
• ECM fails to enter closed-loop oxygen sensor control operation within a specified time interval.
• Engine control or automatic transmission control enters a "limp home" operating mode.
• Ignition is on (KL15) position before cranking = Bulb Check Function. 

Within the BMW system the illumination of the Malfunction Indicator Light is performed in accordance with the regulations set forth in CARB mail-out 1968.1 and as demonstrated via the Federal Test Procedure (FTP). The following page provides several examples of when and how the Malfunction Indicator Light is illuminated based on the "customer drive cycle".

Fig 2: Checking Bulb Function

Courtesy of BMW OF NORTH AMERICA, INC.

1. A fault code is stored within the ECM upon the first occurrence of a fault in the system being checked.
2. The "Malfunction Indicator Light" will not be illuminated until the completion of the second consecutive "customer driving cycle" where the previously faulted system is again monitored and a fault is still present or a catalyst damaging fault has occurred.
3. If the second drive cycle was not complete and the specific function was not checked, the ECM counts the third drive cycle as the "next consecutive" drive cycle. The "Malfunction Indicator Light" is illuminated if the function is checked and the fault is still present.
4. If there is an intermittent fault present and does not cause a fault to be set through multiple drive cycles, two complete consecutive drive cycles with the fault present are required for the "Malfunction Indicator Light" to be illuminated.
5. Once the "Malfunction Indicator Light" is illuminated it will remain illuminated unless the specific function has been checked without fault through three complete consecutive drive cycles.
6. The fault code will also be cleared from memory automatically if the specific function is checked through 40 consecutive drive cycles without the fault being detected or with the use of either the DIS, MODIC or Scan tool.

With the use of a universal scan tool, connected to the "OBD" DLC an SAE standardized DTC can be obtained, along with the condition associated with the illumination of the "Malfunction Indicator Light". Using the DISplus or MODIC, a fault code and the conditions associated with its setting can be obtained prior to the illumination of the "Malfunction Indicator Light".