The MIL is triggered by the car's "On Board Diagnostic" (OBD) system. The OBD systems prior to 1996 are called OBD-1 and systems from 1996 on are called OBD-2. Most of the OBD-1 systems were limited in their diagnostic capability and provided manufactures with real world experience with OBD systems. The OBD-2 systems are fully functional and standardized in their diagnostic code output, meaning anyone with an OBD diagnostic scanner can read the numeric codes transmitted by the cars OBD-2 system. OBD codes are referred to as "P" codes and they start as P0 or P1 and are followed by three additional numbers. The P0 code is generic & the P1 code is a manufacturer code. The next three numbers help identify the possible problem. For example, a three hundred code signifies a misfire. This could be caused by fouled spark plugs, dirty injectors, or low compression. Usually the technician will receive several other codes with the three hundred code, for example P0300, P0302, P0305. The translation for these codes is as follows; P0300 = random cylinder misfire, P0302 = Random misfire cylinder #2, P0305 = random misfire cylinder #5. This would lead the technician to look for the cause of cylinder misfires focusing on cylinders 2 & 5.

When the MIL light goes on the driver sees it as one problem; the technician sees it as one of several hundred possible problems. The codes narrow it down to one or a few problems. When several unrelated codes appear the technician will usually record the codes, clear them & return the car to the customer with the understanding that the MIL will most likely light up again (this generally takes only 2-3 days). At that time the codes will be retrieved again, and hopefully fewer codes will be displayed. The technician will determine which code caused the light to go on by seeing which code triggered a "freeze frame" and check it against the previous data. "Freeze frame" data is a snapshot of what the car was doing when the light was turned on, the data is stored within the OBD memory until it is cleared, and will return until the problem is corrected.

What should the driver do when the MIL comes on? First, the driver must evaluate the situation. If the light is on steady & the car seems to be operating normally, make an appointment within a reasonable time at your convenience. If the light is flashing and / or the car is not operating normally, get your car into the shop as soon as possible because serious damage could result.

When the MIL light comes on, your car is not operating correctly, and until you have some idea as to the problem you could be causing damage to your vehicle. At the very least your vehicle is not running efficiently & is unnecessarily polluting the environment.