On-Board Diagnostic systems use the vehicle’s computers to detect problems with its emission components and other systems. It informs the vehicle operator when a fault occurs and assists the technicians in identifying and repairing malfunctioning circuits.
There are two different types of On Board Diagnostic systems. OBD 1, which operates under manufacturer standards and OBD 2, which operates under a standard set by the Society of Automotive Engineers.
OBD I is a system that identifies faults in the vehicle’s emission and powertrain.
It has been superseded by OBD II, an enhanced On-Board diagnostic system that identifies faults in the vehicle’s emission and powertrain and also tests the vehicles operational system to determine faults that do not affect the vehicles driveability but may affect its safety or emission efficiency.
Both systems provide a common standard that all vehicle manufacturers adhere to. This means that any generic OBD II compatible scan tool can access most vehicle data.
A scan tool is used to access the on-board diagnostic information, and the data link connector allows for the scanner to be connected to the car. This connector is a common size and shape and its location is also standardized, under the driver’s side of the instrument panel.
Diagnostic trouble codes
Diagnostic trouble codes are generic, as are the names used to describe components. Computers also communicate with each other using standardised languages. Therefore, all non-manufacturer specific codes are the same from each vehicle.
Standardized languages also allow the manufacturer to provide specific technologies to the vehicle in order to maintain a level of security in relation to theft deterrent and vehicle immobilisation.
The system reports that a fault exists by a Malfunction Indicator Lamp or MIL located in the instrument cluster or by a scan tool connected to the vehicles diagnostic plug.
The Malfunction Indicator Lamp also indicates, to the driver, that there is a problem in the system. When a malfunction occurs, the Malfunction Indicator Lamp will remain on until the system returns to normal or the fault is repaired or rectified.
A Diagnostic Trouble Code or DTC is then placed in the computers memory. The Diagnostic Trouble Codes inform the technician of the computers opinion of the location of a system fault.
The Diagnostic Trouble Codes are used in conjunction with Flow Charts found in the Manufacturers Service Manual, to assist technicians in determining the likely cause of the failure.
Both OBD I and OBD II monitor engine sensors, fuel delivery components and emission control devices that, if faulty, will cause exhaust emissions to rise.
Simpler OBD systems are normally limited to the detection of an open or short in a sensor circuit.
Whereas OBD II components and circuits are monitored for opens, shorts and abnormal operation.
Diagnostic trouble codes are generated by the on-board diagnostic system and stored in the computer’s memory. They indicate the circuit in which a fault has been detected. Diagnostic trouble code information remains stored in the engine control module or ECM long-term memory regardless of whether a hard or intermittent fault caused the code to set.
The diagnostic trouble code uses a series of letters and numbers to identify which system, component and circuit is at fault. This allows each code relating to a particular system or sensor to be arranged together.
The first part of the code is a letter that identifies whether the fault is located within the powertrain, body, chassis controller, or the communication system (P, B, C, U). The second part consists of numbers starting with 0 or 1. 0 indicates an OBD code. 1 indicates a manufacturer code. The following 3 numbers identify the system and component related to the fault.
On-board diagnostic systems store a diagnostic trouble code in the computer’s “Keep Alive Memory” or KAM. These codes will remain in the vehicle’s memory until power is disconnected.
The OBD II system continually monitors the performance of the vehicles emission system. The malfunction indicator lamp must light when a failure will cause the vehicle’s emission levels to rise. For example, catalytic converter efficiency is monitored and, if its efficiency is out of range, the malfunction indicator lamp will illuminate and a diagnostic trouble code will set.
Monitoring emissions
Under the OBD II standard the vehicle’s computer monitors the emission systems in two ways.
The first, is referred to as “Continuous” where major emission causing faults such as Engine Misfire and incorrect Air/fuel mix are continually monitored.
The second is referred to as “Non-continuous”. This is where checks are made only once each warm up cycle.
Each time the engine is started, the computer checks components such as the Oxygen sensor, catalytic converter and other engine systems are functioning correctly. If a fault is detected the MIL is illuminated indicating that the vehicle needs attention.
If the condition is intermittent and the faulty system operates normally, the MIL will turn off after the vehicle has operated through three warm-up cycles, but the Diagnostic Trouble Code will remain in the computer memory for a set period.
If the fault does not reoccur within 40 drive cycles, the code will be automatically erased but will remain logged in the computer memory as a history code.
