On-Board Diagnostic (OBD) System at a Glance

1. Failure of sensors and devices of some emission control systems may affect engine performance. Reading diagnostic codes and pinpointing a specific faulty device is possible thanks to the use of an OBDI system on the vehicle (until 1995) or OBDII (since 1996). To read diagnostic trouble codes in this system and identify them, special expensive scanners are required, so it is advisable to perform all types of diagnostic, repair and adjustment work in a car service.

2. On models prior to 1995, diagnostic codes are read from the memory of the processor unit by connecting to the ALDL connector, which is located under the driver's instrument panel (see photo). On some vehicles equipped with 4L60-E or 4L80 electronically controlled automatic transmissions, additional transmission failure codes can be read. The processor unit on these models works in conjunction with a programmable memory unit (PROM) and emergency fuel supply unit (CALPAK).






3. On models since 1996, a single PCM processor unit is installed, which provides control of the power unit (engine and transmission).

4. Diagnostic codes must be checked after the SERVICE ENGINE SOON warning lamp on the instrument panel starts to glow.