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8.1 Digifant I On-Board Fault Diagnosis

The Digifant I control unit has built-in on-board diagnosis (OBD) circuitry. The circuitry monitors the normal range of system operating values, detects unusual values and other faults, and stores fault information for later recall. If the system detects a fault it illuminates a warning light on the instrument panel labelled "CHECK".

When operating properly, the light should illuminate every time the ignition is turned on and go out when the engine starts. If the light remains on after the engine starts it means that the system has detected a fault. If the fault is a temporary condition, the light may go out some time after the engine starts, but the fault will remain stored in the control unit memory.


Volkswagen specifies that the first step in any Digifant I system troubleshooting must be to check the OBD fault memory, and to find and correct any faults that may be indicated.

The Digifant I OBD fault memory should always be checked, and any faults that are indicated should be corrected, before checking any of the basic engine settings such as ignition timing or air/fuel mixture. This especially applies to a car that may still be protected by any of Volkswagen's extensive warranties.

There are two ways to access the control unit fault memory. Volkswagen dealer service technicians use a special test instrument, the VAG 1551, to display coded fault information, test system components, and conduct a check of ECU output functions. The tester is connected to the ECU using a set of dedicated diagnostic connectors under the trim plate in the center console. See Fig. 8-1.

Fig. 8-1. Volkswagen special tool VAG 1551 Tester connected to diagnostic connectors to read Digifant I OBD fault codes.

The fault memory can also be accessed using a special jumper cable, Volkswagen tool no.357 971 514E with color-coded white and black terminals. When the jumper is connected, the CHECK light will display any fault codes in a series of flashes. For example, two flashes, followed by three flashes, followed by four flashes, followed by two flashes indicates the code 2-3-4-2. Table s lists Digifant I OBD fault codes. The bold numbers in the probable cause column refer to headings in this chapter where additional tests are described.

Table s. Digifant I On-Board Diagnosis (OBD) Fault Codes

To access fault memory using jumper cable:
1. Turn the ignition On.
2. Connect the black end of the jumper cable to the black diagnostic connector (#1 shown in Fig. 8-1 above). Connect the white end of the jumper to the white diagnostic connector (#2).
3. After five seconds the OBD CHECK light should begin to flash. Remove the jumper, but do not turn off the ignition. Record the flashes.
4. When code 4444 or 0000 appears, then either turn off the ignition or reinstall the jumper to end code readout.

Possible wiring faults should be checked and corrected as described in 8.6 Electrical Tests before replacing a suspected component. Once the fault is corrected, the permanent fault memory should be erased as described below.

To erase fault memory using jumper cable:
1. Make sure the ignition is Off.
2. Connect the jumper cable as described above.
3. Turn the ignition On.
4. After 5 seconds remove the jumper cable. The OBD light should flash 4444.
5. Turn the ignition Off or reinstall the jumper cable to end the procedure.

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