OBD – On-board diagnostics Part 2: Details

Monitoring in the vehicle

The OBD monitors all systems and components that are relevant to exhaust gas. Some components are permanently monitored. Other components are only monitored when passing through a driving cycle – e.g. in the case of defined operating conditions, or when certain operating points are reached. Examples of cyclical monitoring include the catalytic converter, the fuel tank ventilation system and the exhaust gas recirculation. Only if a fault occurs in two consecutive driving cycles will the code be saved, causing the error lamp to light up. If the fault does not recur, the lamp will go out automatically after three consecutive cycles. After 40 faultless driving cycles, the diagnostic trouble code will also be deleted from the OBD memory again.

OBD diagnostic trouble codes

There are standardised diagnostic trouble codes for all vehicle types.
Code structure:
1) The first letter indicates the area in which the fault has occurred:
P stands for drive (Powertrain)
B for body (Body)
C for chassis (Chassis)
U for network systems (data bus)

2) The number in the second place:
0 stands for a standardised OBD code
1, 2, ... stand for manufacturer-specific codes

3) The third position indicates the assembly group of the fault:
1/2 = fuel supply
3 = ignition system/ignition misfires
4 = exhaust gas purification systems
5 = speed and idling control systems
6 = control unit and its output signals
7/8 = transmission/gearbox

4) The last two digits stand for the detected component and the type of fault