Notes on OBD Without VAG 1551 - Start Here

MALFUNCTION INDICATOR LAMP (MIL)

All models sold in California, and some models sold in other states and Canada are equipped with a Malfunction Indicator Lamp (MIL) located on the instrument panel. The malfunction indicator lamp is a means of notifying the operator of problems with the ECM or related circuits and devices. If an ECM related circuit malfunctions while the engine is running, the malfunction indicator lamp will illuminate. This will notify the driver that a problem has been detected, and a trouble code will be stored in the computer memory. On most models, if the trouble is intermittent the lamp will turn off when the problem is no longer present, but a trouble code will still be stored. On other models, when a problem is detected the lamp will light and stay lit until the fault memory is erased, even if the problem was intermittent.

MIL OPERATION

Under normal conditions, the malfunction indicator lamp will illuminate when the key is turned ON, and will go OFF when the engine is started. This is a bulb check and indicates that the malfunction indicator lamp circuit is functioning. If the vehicle is equipped with a malfunction indicator lamp, and the lamp does not illuminate when the key is turned ON, or does not go OFF when the engine is STARTED a malfunction is present, and the system should be inspected.

Do-It-Yourself LED Test Light:

The malfunction indicator lamp can also be used to display any trouble codes that have been stored in memory, however, some models equipped with a malfunction indicator lamp will not display codes by flashing the lamp. These models require connecting an LED test light to the data link connectors in the auxiliary relay station to display codes. Diagnostic trouble codes are read by counting the number of flashes of the lamp while the computer is in diagnostic mode. For a list of the codes and their definitions, refer to Diagnostic Trouble Code Charts. Engine Control System

MIL VERIFICATION

Typical Fault Indicator Light:

To verify that the vehicle is equipped with a functional malfunction indicator lamp turn the ignition switch "ON", and observe dash lights. A "CHECK" light with an engine symbol should illuminate (malfunction indicator lamp).

Relay And Connector Station Locations:

Activating Data Link Connectors:

If no malfunction indicator lamp is showing, leave the ignition switch turned "ON", locate the Data Link Connectors (DLC) in the Auxiliary Relay Station 1 (located in the engine compartment left side cowl). Bridge the terminals of connectors 1 and 2 as shown using suitable jumper wire, (LED test light not needed at this time).

If the vehicle is equipped with a functional malfunction indicator lamp, the lamp will light up while data link connectors are properly bridged and provided the lamp is not burned out. If the lamp did not light when the ignition was turned ON, but did light with the data link connectors bridged, then there may be a problem with the ECM lamp driver circuit, ECM terminal connection/wiring, or the ECM itself. Before suspecting a bad ECM, or if at this point no malfunction indicator lamp has shown, use of an LED test light at the data link connectors will be necessary to extract the diagnostic trouble codes.