OBD (On-board diagnostics), is an automotive word referring to a vehicle’s self-diagnostic and reporting capability. OBD systems provide the vehicle owner or a repair technician access to state of health information for various vehicle sub-systems. The amount of diagnostic information available via OBD has varied widely since the introduction in the early 1980s of on-board vehicle computers, which made OBD possible. Early instances of OBD would simply illuminate a malfunction indicator light, or MIL, if a problem was detected—but would not provide any information as to the nature of the problem. Modern OBD implementations make use of a standardized digital communications port to provide real-time data in addition to a standardized series of diagnostic trouble codes, or DTCs, which allow one to rapidly identify and remedy malfunctions within the vehicle.

The regulatory intent of OBD-I was to support auto manufacturers to design reliable emission control systems that stay effective for the vehicle’s “useful life”. The hope was that by forcing annual emissions testing for California, and denying registration to vehicles that did not pass, drivers would be likely to purchase vehicles that would more reliably pass the test. OBD-I was largely ineffective, as the means of reporting emissions-specific diagnostic information was not standardized. Technical difficulties with obtaining standardized and consistent emissions information from all vehicles led to an inability to implement the annual testing program successfully.

OBD 1.5 refers to a partial implementation of OBD-II which General Motors used on a number of vehicles in 1994, 1995, & 1996. For example, the 94–95 Corvettes have one post-catalyst oxygen sensor (although they have two catalytic converters), and have a subset of the OBD-II codes implemented. For a 1994 Corvette the implemented OBD-II codes are P0116-P0118, P0131-P0135, P0151-P0155, P0158, P0160-P0161, P0171-P0175, P0420, P1114-P1115, P1133, P1153 and P1158.
This hybrid system was present on the GM H-body cars in 94-95, W-body cars (Buick Regal, Chevrolet Lumina (’95 only), Chevrolet Monte Carlo (’95 only), Pontiac Grand Prix, Oldsmobile Cutlass Supreme) in 94-95, L-body (Chevrolet Beretta/Corsica) in 94-95, Y-body (Chevrolet Corvette) in 94-95, on the F-body (Chevrolet Camaro and Pontiac Firebird) in 95 and on the J-Body(Chevrolet Cavalier and Pontiac Sunfire) and N-Body (Buick Skylark, Oldsmobile Achieva, Pontiac Grand Am) in 95 and 96 and also on ’94-’95 Saab vehicles with the normally aspirated 2.3.


For ALDL connections, pin 9 is the data stream, pins 4 and 5 are ground, and pin 16 is battery voltage.An OBD 1.5 compatible scan tool is necessary to read codes generated by OBD 1.5.Extra vehicle-specific diagnostic and control circuits are also accessible on this connector. For example, on the Corvette there are interfaces for the Class 2 serial data stream from the PCM, the CCM diagnostic terminal, the radio data stream, the airbag system, the selective ride control system, the low tire pressure warning system, and the passive keyless entry system.An OBD 1.5 has also been used on Mitsubishi cars of ’95 ’97 vintage, some 1995 Volkswagen VR6’s Buick Riviera of 1995 vintage and in the Ford Scorpio since 95.

OBD-II is an step up over OBD-I in both capability and standardization. The OBD-II standard specifies the type of diagnostic connector and its pinout, the electrical signalling protocols accessible, and the messaging format. It also provides a candidate list of vehicle parameters to monitor along with how to encode the data for all. There is a pin in the connector that provides power for the scan tool from the vehicle battery, which eliminates require connecting a scan tool to a power source separately. However, some technicians may still connect the scan tool to an auxiliary power source to protect data in the unusual event that a vehicle experiences a loss of electrical power due to a malfunction. at last, the OBD-II standard provides an extensible list of DTCs. As a outcome of this standardization, a single device can query the on-board computer(s) in any vehicle. This OBD-II came in two models OBD-IIA and OBD-IIB. OBD-II standardization was prompted by emissions requirements, and though only emission-related codes and data are required to be transmitted through it, the majority manufacturers have made the OBD-II Data Link Connector the only one in the vehicle through which all systems are diagnosed and programmed. OBD-II Diagnostic Trouble Codes are 4-digit, preceded by a letter: P for engine and transmission (powertrain), B for body, C for chassis, and U for network.

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